We had already put the twist throttle on and we were thinking about how to mount the motor.

I really wanted to keep the pedals so we had to make sure it didn't get in the way.  We got a couple other bikes from the dump for parts - a better kick stand from one and a replacement inner tube from another.  We figured that the easiest way would be to use a friction drive, to make a small wheel on the motor rub on the back tire.  My dad designed a motor bracket and I helped him machine it on the bridgeport.  I also helped turn a motor wheel on the lathe.  We had to make a strong mount for the heavy batteries, so we used steel angle that was welded together to mount the two batteries.  I did half of the welding on the battery mount which was really fun - I want to do more welding!  Once that was done, I soldered the wires together and we found that the hall effect throttle didn't let the motor stop when you let go of it!  We had to add another switch to the throttle to make sure that when you let go it stops the motor.  With that figured out, we had some fun with it after we made a few adjustments.