You can only go so fast before you have to make it conform to all the motorcycle regs. Blah.
4 stroke has the edge re: noise, emissions, fuel efficiency, longevity.
I can see what I need to of the drivetrain. It likely has a centrifugal clutch which then leads by chain- it appears at a 1:1 or close to it ratio- to a jackshaft which then leads by chain- it appears to have a 4:1 reduction- to a fixed sprocket secured to the wheel by bolts threading through gaps between the spokes, with probably some sort of multi-piece disk on the back side to clamp them.
Edit:
http://thatsdax.com/TITAN_INSTALL_PAGE_1.html
Ohhhh, I see. The engine has a built-in gear reduction before the clutch!
I'd long thought that I'd try to put a v-belt pulley on the rear wheel and forego clutch and trans, either by pivoting the whole engine or by using an idler. Then easy pedalling capability would be retained.