Go for the carby job mate. I've got a 240 and they add low- and mid-range, but not top end. This is to be expected given that the carby and porting, etc, is designed to feed a smaller cylinder. Boring any engine out usually just shifts the torque curve lower down the rev range unless other mods are done to improve the breathing too.
If you've already got a 220, the increase in capacity moving to a 240 (which will not be a full 240, but 23x) is negligible and not worth upsetting the engine design so radically. The iron sleeve and (reputedly more expansion-prone) Wiseco piston both seem to make a 240 run hotter with a tendency to seize, and it's hard to get them jetted to idle nicely. It's not worth it unless you are desperate.
Craig