project bike

[DEATH_INC.]

If yer looking at slightly bigger engines,my vote's for the xr200 2 valve engine too.Compact little buggers with reasnoble poke.
A rg 150 engine would be a goer too,or a 175-200 two stroke (it or kdx) engine.

[FzerozeroT]

WTD smallest lightest "motor" capable of reliably powering a highway going minibike, cylinder orientation preferably vertical.

how small is "minibike"?

www.minimoto.co.nz

if it's that small, you won't be able to fit the tuned length of the exhaust for a two stroke, so go for 4 stroke, xr200 sounds like the best (cheapest vs lightest) option so far, wrecked scooter will be easier with your idea for swingarm as it will include wheels and electrics to suit a minibike, just need to weld a new frame up