I actually have a GP20 mech that's already installed sitting on the bench. It would take about 5 minutes to swap the motor into your frame... ten if I have to do a little milling. Or if you're satisfied with the way it runs, we stick a shell on it and go. It's got a 123 in it, I think...
If you want to go fresh, get one of those TCS split jobbies. Just put one of them in for Brian, literally took about 10 minutes. I only had to modify the numberboard casting because their LED pod sticks out a bit.
Lee