This will save you a lot of work, and will also give you a feel for the differences between the two editions. The 5e Monster Manual is quite comprehensive, and includes sample NPCs at the back. You can alsoalthough you may have to be a bit creative here - so there are no Neanderthals in the MM, but there are "Tribal Warriors".
- useUse positive armor class: SinceStarting with 3e D&D, armor class goes up rather than down from 10, so AC 3 in B5Basic D&D = AC 17 in 5e.
- decideDecide some Strength, Dexterity and Constitution scores on the fly and apply the modifiers to combat rolls, hit points and saving throws. Don't sweat this too much - it's really just a case of deciding on a number from -4 to +4 to give the monster strengths or weaknesses.
- giveGive the monster or NPC skills or special abilities which make sense for its role in the adventure, so if it is meant to surprise the PCs, give it Dexterity (Stealth) +4 or +6
- convertConvert saving throws using the PHB. So if a monster's stat block in B5 says it has save F4 (Fighter level 4), give it proficiency (+2) in Strength and Constitution saving throws, as per the 5e PHB Fighter entry (PHB 71).
You can read up on these in the 5e core books you have. Most of what you need to start is in Chapter 7 of the PHB, and the DMG gives more detailed advice if you need it.
I would interpret this in terms of a DC 15 Wisdom (Perception) check to spot - so unlikely to be spotted with passive perceptionPerception, and then a DC 10 Dexterity Saving Throwsaving throw so as not to fall in - about even chances, with high Dexterity making a difference. Note that in 5e both of these rolls mean trying to roll the target number or Note that in 5e both of these rolls mean trying to roll the target number or above on a d20.above on a d20.
- Enjoy!You can get more detailed with the DMG
An alternative way of dealing with traps in particular would be to take the most similar ones from the examples in the Dungeon Master's Guide, so 'Hidden Pit' (DMG 122) seems the best fit here.
The same goes for magical items - either leave them as they are, or take the nearest alternative in the DMG.
- Enjoy!