Sounds like an coaster brake problem. Did you ride the onyx before you put the engine on? Maybe the rear coaster brake is what's wrong.
Oh wait, I think I know what you did. The rear wheel, does it spin freely with the clutch in? I think you may have made something tight that shouldn't have been tight, and i think it probably involves you coaster brake. I don't know exactly how they work.
If it's not that, check to see if anything is rubbing near the rear tire bolts.
The bike goes faster than ten MPH stock, so you must have messed up that coaster brake assembly somehow.