How much larger engine will fit your Doodlebug minibikeThe stock 2.5 HP is just plain weak. Thanks?

I would just go ahead and get a 6.5 hp engine from HF. They are about 120 to 180 dollars depending if they are on sale. I have one on mine and it has a decent amount of power. Also, if that still isn't enough for you; you can always buy performance parts for it.