Actually, there is a performance difference. The 21 and/or the 25 will certainly work, but the older 23.6cc engine is the best. This is because of the better porting on the older 23.6 cylinder. The one I'm currently running is the 23.6cc stroker that Keith mentioned. I made it out of a 25cc crankshaft and a 23.6cc cylinder, piston, and crankcase. I had to make a spacer to raise the cylinder to make up for the increased stroke. I also ported it and modified the head as advised by Keith, and it runs great! There is a thread here about a guy who just put together a Funtana with a Kioritz, and I think he used a 21cc. You might look into that thread.

Good luck,
AV8TOR