Try a CAG. Pocket bike engine. You then only need the proper drive sprocket(if you want to keep your current chain) and you're good to go. Yes, you'll have to figure out mounting it, but it's not that hard.
The benefit is that the quality is night and day difference and there are aftermarket parts out the wazoo compared to the HTs. They are also smaller and lighter than the HTs. I got one and tore it down first thing. Everything I found was better. I'll never go back.
Life's too short to ride slow.