HV,
while this is rack to cassette as you say, this isn't precisely what "kerf" and others have in mind.
this is single ratio from the engine and multi pedal ratios.
most others would like to have multi engine ratios. i believe it can be configured, BUT, as i found with the frame mount shifter build, the bike chains and deraileurs will only take so much power before you have lots of issues.
the best way to do it, no matter what engine, would be to use 1/2" x 1/8" chain from the jackshaft to the cassette, you can't derail it for remote shifting but you can use an adjustable tensioner/ guide. move the chain by hand and adjust out the slack again.
Surly makes some very good single speed cogs in the 16T to 22T range that work on Shimano cassettes and could be stacked for maybe 3 ratios of engine power, hopefully leaving enough room for one pedal ratio.
you would just need to choose climbing or speed ratios and go. this is greatly simplified by using the 5:1 scooter tranny, since it reverses the rotation getting you over to the right side and allows the use of smaller sprockets to achieve the secondary reduction.
my finding is that with the power and RPM range of the GP460 i can get along fine with just one well chosen engine ratio. the novelty of lever shiftable ratios is pretty much wasted on this engine since it's comfortable with turning 15,000+ rpm ALL DAY.
EDIT:
or you could use a double freewheeled jackshaft driving from either engine or pedals to a multi speed rear hub. which would allow you to use real big chain, extra power and no derailleur. Hmmmmmmmmmm.
thanks,
steve