More torque?

[WTFboom]

I have a Baja DB30 Doodle Bug and it does not have enough torque to get me up small hills. I have to get momentum to conquer small hills. I have a 6.5hp engine that i was going to use for a go kart i was going to build. The speed of the bike know is fine. It would even be OK if it went slower. I have removed the governor and the throttle limiter screw. What i want to know is should i put that new engine in (i would have to get a new clutch bell, a 10T or a 12T but the 12T needs a #40 chain i have a #35). Or i can get a new rear sprocket (the stock is a 70T). I can get sprockets all the way up to 94T (I just have to drill my own mounting holes). Or should i do both to keep the speed down and torque up. So, what is the setup i should use?