RE: Best .21 engine
I don't know how it stacks up against modern engines, but you can still find the K&B 3.5cc rear exhaust engines. I've got one on my Shrike 10. I've also got a NIB spare that's still looking for a ride. I don't recall the exact numbers, but it's turning a 7x8 somewhere between 18 and 19k. And if that's not good enough, there's some boat guys who know how to tweak the timing to get more power out of them. It's the same basic engine that K&B uses on their outboard engine.