I'm sorry, I made that confusing. You can get them in different sizes, yes. But the one cloeset to 72 is 73mm. So, there is no 521cc, you would have to go up to a 535cc.
okbeast, according to all of the builders/racers that I've spoke to, there are no more reliability issues with the 10mil than the 4mil. You just have to upgrade on transmission parts. Availability for parts is just the same as any other aftermarket Banshee cylinder. They've been making Cheetah cylinders longer than the Cubs. The cub uses stock Banshee stuff, and the Cub uses stock Honda CR intakes/reeds. The heads come with removable domes, and you can get them with either a small flange or big flange exhaust outlet. The Cheetah uses Weisco pistons, rings, wristpins, and bearings. The only thing different would be the power valves. And you can get those pretty much at any CP Industries dealer.