Forget the extra head gaskets, Cubs have low compression. Substitute a Cox RTF needle assembly for the Cub one, clean up the crankshaft port and bypass entry holes and there you are. I have used up to 60%+P fuel with good results on small props and no failures with these mods. Oh yes, I also did a bit of grinding to improve crankshaft balance, chamfered the venturi entry a bit and rounded the exit hole from the crankshaft. Just touch up stuff, probably not much of importance.
After dealing with the hideous needle valve design, cleaning up the flow and balance seem to be the key to Cub performance. Followed by the fact that it has low compression. Hmm, unless you can’t get Nitro, is this a problem? 😜