My first useable engine was a Cub and it ran just fine. I have had a few since and they all run. I even hopped one up to at least Cox Medallion standard or better. Had it on a 1/2 A Spacer at Taft and coming back from a flight a fellow modeler was flabbergasted and said “That’s a Cub?”
They can run just fine and can be hopped up with a few simple mods and really hot fuel. The compression is low, the fits tend to be loose, so they can handle the Nitro just fine. A hot battery for starting is a good thing. The best mod is to substitute the needle assembly from a Cox rtf backplate for the Cub one. (more later) Orient the exit hole about 15 deg below horizontal for optImum fuel draw.
There is a weird ledge in the port in the crankshaft that obstructs flow. I grind that smooth. I also clean up and smooth the entry to the bypass ports. If truly inspired, the crankshaft balance can be improved. With all this done you can amaze your friends with what a Cub can be.
An ideal model for a stock Cub is the 1/2A Snapper. Full pattern capable on very light lines. Not quite so good would be the Brodak 1/2A kits unless you lightened them up a lot! Perhaps a Goldberg Jumpin Bean is in the ballpark.
If all you want is to go round and round, of course all the ScientIfic and Blackhawk replicas would be fine.
The Cub needle really sucks. It pretty much works by a blunt end shutting off the fuel hole. Almost an all or nothing approach making perfect adjustment impossible. Not the standard smooth taper giving easy control of the metering. Chamfering the top of the venturi would be a good thing too.