Is there an RPG system based on Robert Asprin and Jody Lynn Nye's Myth Adventures series? I'd also be interested in any adaptations of the setting for a pre-existing system, like GURPS.
I'm not familiar with the RISUS system but I have found that any magic based RPG's work well in combination with the MAGE (http://www.white-wolf.com/) rules. They are extremely flexible, allowing for many on-the-fly decisions, limited primarily by the imagination of the players.
Since magic in the world of Myth Adventures tends to rely on many mental gyrations (for instance when Skeeve disguises himself or the group by imagining their faces being replaced with something else), the Mage rules fit by not requiring the player to have pre-memorized a specific spell from a spellbook. Rather the player needs to be able to describe what the intent of his spell is and how it affects those around him. Mostly it's about making sure the magic is convincing.
Mostly it boils down to how inventive your players are. If they are 'by the book types' then a system that has pre-generated tables of spells would be best. But if they are quick-witted , capable thinkers then something free-form like Mage would be ideal. You just have to gauge the type of players you're working with.
In keeping with what Wraith808 pointed out below I'm adding a few details concerning how I started setting up the campaign I was going run some years ago using a mishmash of MYTH and MAGE:
Tananda - The Beautiful trollup assassin - Essentially you could use all the traits from the Akashic brotherhood to create her. The Akashics are mages that focus on fighting in some way or other. Tanda is a shoe-in for this kind of character.
Skeeve - The 'hero' of the series - Closest match to his talents are the Order of Hermes. They are seekers of knowledge and tend to surround themselves with musty tomes and ancient writings. Skeeve in the very first book seems to think this is what a real Wizard would look like and thus this prototype fits him very well.
Aahz - Skeeve's sidekick, or arguably the center of most of the adventures they have - Aahz is unfortunately without powers for a great deal of the series but if he had power he would be classed as a sort of outsider with abilities from the Akashics, Verbena, Euthanatos and Order of Hermes. He's a powerful daemon.
Again, a lot of the game world itself you are really going to pull from your own notes on what is important within the books that Asprin wrote. Lay-lines can simply be a location where your characters get bonuses to their spellcasting ability that doesn't incur as many penalties. As I wrote below, there are several ways that paradox can be overcome because many of the worlds Skeeve and Aahz visit accept magic as a daily occurrence. The Bazzar at Deva is one of location in particular where you;re players could have all kinds of fun without wracking up any paradox.
After reviewing some old character sheets I see I forgot to include some basic thoughts on character creation mechanics.
The Mage character sheet is very simplified, geared more toward story telling than having to roll dice for every option under the sun. The sheet is broken up into Character, Attributes, Abilities, Spheres and Advantages. That's it! This allows the GM to be very creative in how an encounter can be worked out.
Character focuses on simply naming the players character, giving them a nature and demeanor, setting their tradition, essence and experience. When I was contemplating running a MYTH game I ignored tradition because that really only matters if you're going to be encountering other traditions where politics of play might become necessary to gauge. Obviously in the MYTH world there aren't any set traditions (unless you can think of a way to create a one to one relationship between different factions of said world and the traditions, though I never found an easy way to pull that off). Nature and Demeanor really are the same as D&D alignment the whole Chaotic/Lawful/Neutral/Good/Evil thing. Essence is the pool of magic that the character has to pull from and Experience I hope is self explanatory.
Attributes cover physical, social and mental skills. How Strong a character is, their charisma, perception, etc. These are all factors that easily fit with any given character in the MYTH world. Skeeve for instance is a rather intelligent young man yet not particularly strong of dexterous. Tanda on the other hand Is highly dexterous and charismatic but not particularly intelligent (though she often shows a high wisdom in certain instances).
Abilities are where you confer certain talents, skills and knowledges to your characters. Aahz is a good example here because though he is dispossessed of his magical abilities he has many other talents including Awareness (5 because of his incredible hearing), Brawl and Dodge, Streetwise and Intimidation. Aahz further has skill in meditation, survival and in certain instances shows a good knowledge of technology (though that one might be better left out depending on your point of view of certain devices in the world of MYTH). Aahz also has a huge head of knowledge including Academics, Cosmology, Enigmas, Linguistics and Occult.
Finally the all-important Spheres. These are the actual magic that is so wonderfully blended into the Mage world and thus easy to adapt to a MYTH campaign. The spheres break down into types of magic that can be cast: Correspondence (i.e. How Skeeve teleports everyone to Deva from Klahd, etc.), Entropy - magic that breaks things down, Life - the control of life and by extension death, Matter - care to shape the world to your will? Mind - Oh yes, you WILL do as I say! Prime - pure energy and the keys to with it what you will, Spirit - Sure he's dead but that wont be a problem. Time - never be late again!
The advantages section is basically to keep track of what effect the spells you cast are having on the world around you and the caster themselves. For me, I only used the Willpower, Health, Quintessence and Equipment sections as the backgrounds, merits and flaws, resonance and Arete really didin't seem to apply to anything in the MYTH world.
And that's it. Originally I was going to plop my group into the Bazaar at Deva and let them go wild. Telling them that there are games in various tents, interesting smells, and all manner of craziness, eventually pulling them into a fight with a stall master who would throw them into a world where the laws of physics were backward.
Too bad I never got to run the game, I think it would have been epic. I hope you have better luck with your group!