There are none, but it's possible that you don't actually need to increase scores. As far back as his proto-D&D Blackmoor campaign, Dave Arneson used a method where he would roll from two to five d6 (depending on the difficulty of the task) and roll under some relevant characteristic, plus 1 point for every two character levels. So if you were a 10th-level M-U and had to roll 3d6 vs., say, a 9 STR, you would actually need to roll 14 or less (9 + 10/2). This was a fairly common ad hoc method of resolving tasks that weren't already covered by the rules.
And depending on what edition you're thinking of, bonuses for stats don't make that much of a difference, anyhow. But even if your preferred edition gives you a bonus to your HP per level for a high CON, for example, consider that, as you level up, you're getting more HP in the first place. Not all of those points represent extra toughness, certainly, but some of them might. Also, in situations where a high attribute gives you a bonus to rolls (such as the STR bonus to hit) consider that the same effect results from leveling up and needing to match a lower number in the first place. Getting a bonus to your STR in addition to having a better attack roll would be double-dipping.
Retaining your original stats gives you a good role-play opportunity as well; someone who is "naturally" strong might be big and beefy-looking, but someone who succeeds because of experience might have deceptively smaller but better-toned and -conditioned muscles. Or consider someone who is intellectually gifted versus someone who studies hard, or who has learned the ways of the world through personal (and sometimes painful) experience. As your player advances, you can have fun playing out these differences.