What did you do decide about going to a 125 motor? I faced the same dilemma. I had a 100 motor to start with but wanted 125. I found out that the cases need boring in order to receive the bigger mouth of the 125 cylinder. I ended up just getting 125 cases, cylinder, piston and head. I'll swap the 100 guts into the 125 cases. So, I just ended up with a motor that is 90% 125. It would be easier to just start with a 125. The 100 had too many roadblocks to get to 125 (need 125 crank for 15mm pin, need cylinder and piston of course, then head to match, now cases to match the cylinder). Well, at least the 100 motor only cost me $30. The trans is mostly the same. The kickstart shaft is smaller on the 100.