RE: What is the max prop RPM?
My understanding is that car engines have raised exhaust timing for high rpm, so their rated rpm is only meaningful if turning a little gear. By raising the exhaust port, you sacrifice the amount of time that the piston gets pushed by the expanding gas, so there goes your torque for doing more work [like turning a prop]. These engines are also equipped with big ports and passages that don't breathe right with too much load, not enough rpm. Bolt on any usable sized prop and the car engines turn into very expensive paper weights. If you look at a Picco .21 [the only .21 I've ever owned] it looks about the same size and weight as a .35 [so now you have both a heavy AND guttless paper weight]. If your .21 is going to be well suited for aeroplanes, it's possibly one of those that I was told was developed for airplane racing and speed before I got very far into the hobby. For example, I just finished building a P-51 kit that was turned out just for .19-.21 engines.
If you have any doubts about props and rpm, Steve Wilk at Eliminator Props has good advice and carbon props that wont let you down. I'll bet a hot .21 can turn a 7x6 or 7x7 pretty good. A 7 inch diameter prop will fly a 2-2.5 pound speedster pretty good. Hopefully this .21 of your's works well in airplanes.