I rode my N300 on forest service dirt & gravel last week at 35-70 MPH... The engine deserves a good rally type chassie/suspension. 3-4th gears are awesome at my +1 CS and 17" wheel gearing. Smooth response, enough torque and almost zero vibes. My bro had his piped/jetted KLX250 there too and said he'd take my 300 engine over his all day long, just slip it in his frame/wheels/suspension.
I'm thinking a older KTM 620/640 LC4 Adv roller for mine...
Figured you needed a boot in the rump to get started!