If you absolutely have to go with a 79cc your best bet is to look for a well respected automotive shop that specializes in rebuilds. While not a car, an experienced machinist will have no problem taking precise measurements, know how to get maximum power, where to locate parts, fabricate parts himself, or offer the exact advice your looking for. If he doesn't have the time to work on a small engine he probably would know the next best place to go. Expect to spend much more than you thought.