: one of those XL(?)350's that Honda made would mock-up OK, its a vertical single so the engine would look close as you could get for a reasonable outlay. everything else is worth too much that i can think of.. i've seen a home-made chair with a leaf spring from a fiat topolino used, and some heavy pipe. the dusting style chair was made out of marine ply with a solid timber bottom bolted to the mounts he'd welded on. i think the fuel tank , seat and side covers would need replacing with more sympathetic period style items, and the front guard (mounted at the right height) i'd also put gaiters on the forks and drop the bike at the front a bit by loosening the triple trees and forcing the stanchions up an inch or so will lower the front.. heaps of olive drab and it would look passable to the crowd, as you know nobody else will be fooled though.

PS i think it also needs a headlight/nacelle and tail light,  some trailer light might do the tail light job.