www.treatland.tv has 70cc kits for the chappy
the lb50 is known to be sluggish! Part of its charm...haha
It is a two-speed auto, with a high range and a low range, you are in high range, correct? As far as motor swapping, it should work, never checked the mounts personally, but imagine that other than the pedals on the lb50, the only essential difference is the motor.
In most states, if you go above 50cc it is technically a motorcyle and the operator and machine must be licensed as such, just a heads up in case you hadn't thought about it yet. The chappy is great, love mine, if it were currently running!!!