I would sugest, if the moter is happy to go onto the self proppelled base, then move it across, looks to me like you are right about the crank length.
Any series 80 cover that does not bolt to the base should do, or you could always use the white cover as a guide and make the little square holes to plug the grommets into.
With the diff, the only time my wheels lock up in reverse is when the cable is engaged (mine is the MKI Though) as the effort to pull the engine cannot be generated, but when its released the clutch in the gear box just lets it roll (a little heavy though, you can feel it) so maybe your clutch has jammed, and the gearbox it trying to turn the engine (this would not happen when going forward, even when jammed, due to the overrun clutches in the wheels) it could be the box lever, or cable or it could be inside the diff.
I'll have a look at the MKII diff on my Power Plus (no belt) and see if i can work anything out there about the locking.
