Coolster Challenger chop and mod
 

I'm new to karting/buggy mods but came across a Yerf 3002 with some monster tires on it and a Coolster Challenger 125CC with an engine that's toast and needs new everything and refurb. The front end on the Yerf was pretty beat up because of the size of the 23" front tires and probably some abuse on mountain trails. The Comet Series 30 was missing (of course) and a pretty worn out Tecumseh 6.0 was on it with a Cent. Clutch. How that was supposed turn the 25" tires in the back I do not know. So anyways comparing the two frames I noticed that the Coolster was a better built frame with thicker tubing and welds. Since my son and I want to do a fun an unusual project together I thought it would be cool to drop the rear axle of the Yerf that somebody had modded with a wider 1" axle and put that on the Coolster. I think we'll graft an ATV front end up in the front to handle the larger 23" tires from the Yerf as well. Making some loose measurements the Coolster is 5" wider but has a wheel base that is 5" shorter. I'm 240 lbs and I can sit in it relatively comfortably. I will definitely need to modify the rear engine mount area so I'm wondering if a 6.5 HP with the Series 30 TC will work for going up hilly trails of if I should go with an 8 or even 12 HP with a Series 40 for the power and torque. I know the geometry of the steering is going to be wacked out with the shorter wheel base and much larger tires. A pic of the Coolster with the larger tires mocked up with the frame sitting at the approximate height makes a cool-looking buggy. I'm not sure about the steering and handling though. Any thoughts on this?

What happened to the engine out of the coolster?

Sat upright with the carb opening filled with water for about 6 years. Everything pretty much rusted solid internally.

Did it have a 150cc or 250cc engine ?

125cc

I would go back with that engine, the drive train is already setup for that engine. That engine is rated 9HP, 5HP at the rear axle but there are plenty of parts for that engine. With good parts and properly set up you can more than double the HP. The preditor engines will have more HP out of the box but it will be low rpm high torque. The GY6 engine will give you more power for less money.