I did not do it out of neccessity. 2100 cranks are cheaper to find than 3120 cranks. As for the 2100 being rpm limited. They have a circut in the carburetor that feeds extra fuel at a certain rpm and slows down the saw. It is easy to bypass and almost all 2100s I have seen have already had it done.
Busmech, yes my brother Jon did do this and he went all out. Instead of using a short piston he had a machinist cut the combustion chamber up and used the 2100 piston. His cylinder is ported, it has a 090 carburetor and the saw is piped. He said it beat any 2100 he had (maybe not any I have, we haven't tried). Anyway I wanted to see what all the excitement was about. I will fiddle with it some more next week and we will see. Mike