If you're considering unit construction (engine and gearbox together), why not stick a british engine in a relatively modern japanese frame? You're going to have to run modernish jap suspension if you want to enjoy proper offroad anyway. It's then your call whether to stick with modern style plastic parts (mudguards, tank, headlamp etc), or go with heavier metal parts for a classic look which will really confuse anyone trying to work out exactly what your bike is. Maybe it's possible to track down early plastic parts but surely they would be expensive.
|