Plugin, I wouldn't worry to much. Build the Mojo 60 ...try to keep it light and use the 91FX. If you are unhappy with it THEN go through the hassle of buying a new expensive saito 115 ($359). If you buy a larger engine than the 91 FX, I think you will start to run into weight issues. Maybe a .91FX + the tower muffler will give you the oomph you need