I bought a ST1100 transmission off of ebay for less that I could get just the transmission output shaft by itself.
It took about 30 minutes to get it all apart and remove the shaft. This leaves my original motor and transmission intact, which should make it easier to sell. Here is the pile of leftover gears.
Now I need to take some measurements of the shaft and get it machined for some bearings and a pulley.
I put a piece of tape on the shaft, and on the tire, and spun the shaft while watching the wheel spin. I measured a 2.83:1 ratio for the rear gears and a 6'9" circumference on the tire.