I hate to say it, but the most effective and affordable engine mods are probably your own. Commercial engine modders really only do very basic and very simple modifications, and those run about at least $100 per engine.

The best thing to do is save your worn piston and cylinders when they wear out, buy a decent rotary tool (like Dremel) with a variety of cutting stones and bits. Practice on these junk cylinders and /or junk engines, then when you are adept enough, mod a new engine when you feel comfortable.