Re: shift kit
Have you thought about adding a "jack shaft" and adding your dive line to the main drive side? With the help of a canter-lever, you could actually bring the drive into play on the same side and still use your complete gear array.
I have chosen an engine with a pull start to make this transition simpler, and completely removed the peddles all together. Haven't thought about using the forward gears until just now, ooppss there goes the light bulb, just popped from being over loaded.
This aught to be real fun adding the three forward sprockets to jack shaft drive unit. 8-)