You can pop the piston up until you get 0.030-0.040" from the top ring, then you will start burning the rings. Remember this-the 10 hp. uses a 2.75" stroke and a different pin diameter than the 12-18 hp, which uses a 3.25" stroke. The longer the rod you can use the better, (but you will have to get a different piston-Chuck Vogel sells all the stuff you would need if you don't want to make it yourself.) I think the 12-18 hp. with a stock stroke falls off on performance when you get past 6.00" in rod length. But the longer rod will actually slow down the piston slightly at TDC and BDC giving the air column more time to get in to the cylinder once it is moving. As far as fins on the flywheels, they really aren't necessary unless you plan on driving them around the pits alot, when going down the track the engine will build more heat than what the fins can dissipate, plus you may run the risk of spinning one off. Some associations have actually banned fins on the steel flywheels. Sounds like you are on the right track though, just test your parts exhaustively on the track before you sell them. One bad part or someone getting hurt can really sour your business plans.
Good luck!
