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.
With out cracking the motor open you can do a header, air filter and re jet for pretty cheep. Start there first. You could also file or replace the timing key and add a little more timing to the motor. See if you have any adjustment out of the governor too because you might be able to get an extra 100-200 RPM out of the motor.