Re: Recommend an 80cc motor
If you put 4 HP on that you wont be able to keep the front wheel on the ground!
Would be fun!
Your going to pay more than $200 for anything close to 4 HP.
Depends on how much you do your self.
If your old motor is a standard 38 mm stroke slant head motor, my advice would be to start with a 40mm stroke GEN4/GEN5 motor from Dax or MBR, Give it a little more compression and a porting and you will likely not be disappointed.
If still looking for more, then try a decent expansion chamber.