Doing a 3" bore would be a waist with out doing a full engine build, cam, carb, intake, exhaust, valve train upgrades ect... pretty much every thing replaced. If your not racing and just looking for more torque than a predator 420 if it would fit would be a better choice. If the motor is all stock the over bore is the last mod I would do. If your looking at getting more power I would look at doing the proper mods to eliminate the governor "Flywheel, rod and springs" if you want more power I would mod carb or replace with something like the mikuni and a header or at least re jet and do a header. If you still want more power after that swap a cam in it and then move on to head work " milling for compression, porting and larger valves". For now on the cheep you might try doing a header, re jet carb, air filter and a timing key. If your willing to dig in to motor get some gaskets, take the head off and try porting it your self.