When I was about 14 (1967ish) got a beat Bronco from a neighbor and put n REO reel type lawnmover engine in it. That frame was just made for doing that.
I must have cut the crank arms off 'cause I mounted up a big pulley on the off side from the chain and tried to get a pulley ration to pull with the stock chainring and rear sprocket. I just didnt know enough about gear ratios and all back then so it never really worked. Got my first motorcycle at 15 and the project was over.