Would be my guess too. Ad says the engine and trans came from car but it still needs rear wheel gearbox. Which to me sounds like he was doing a conventional(ish) shaft drive and never got the final drive sorted.I'm thinking he probably welded the diff inside the fwd trans bolted to the back of the motor then was gonna run a jackshaft of some sort to the motorcycles drive shaft. If not that's how I would finish it off?
Also worth noting as unfinished is the "hydraulicly articulated steering" - for a two wheeled steering system, natch.