I splurged last month, and got myself a Lifan 125cc engine for the Trials bike. There were too many hills and adventures the original 4hp 49cc pushrod motor just couldn't handle. So thanks to Tbolt, I had a new 125cc motor in two days. I forget the exact power numbers, but it was something like double the HP, and four times the tq, at lower rpm numbers.
I had to fab up a new exhaust, which I had been wanting to do anyways. I tucked it up high, and much closer to the frame:
It runs great. I can doddle along at nearly a dead stop in second gear at low rpm, and have more than enough oomph to lift the front tire up and over curbs with a slight blip of the throttle. No more 8,000rpm in first gear going up a gentle hill.
I still need to fit the skidplate and wire in an off switch.