I've recovered the engine back to my house and had a tinker.
And all was going so well, well sort of.
After an entire evening of riding in the heavy rain I fetched the Z482 from a friends house where it had been nesting under a garden table wrapped up in a WWII tent carry bag and a blue tarp. It was a round trip of some 150 miles and it rained for much of it. Here it is in the spare room of my house. There are no photos of it on the new trailer (with mudguards) because of the wet and dark.
The main problem starting it was low compression and after turning it by hand and putting my ear over the inlet and outlet manifolds I concluded it was the valves leaking which leaves very little options other then whipping the head off ASAP.
First was the rocker cover.
Then the head. It was simple enough and everything was not seized solid or rusted away.
There was some loose debris in the combustion chamber and after removing the valves I could see they were very worn but still serviceable with a bit of lapping in. I'll do this my self because if I go to a head shop they will nag me to buy it new valves which I'd rather avoid until the engine has proved its self reliabable.
If you look at the top left corner of the above image you'll see the crank end of the old R80 engine and if you're paying attention you'll spot the remarkably similar bolt pattern.
This required immediate investigation and after removing the flywheel I started getting excited. Just look at the BMW flywheel washer top centre of the above photo.
I then placed the R80 flywheel over the Kubota flywheel and the bolt holes lined up and were indeed the exactly same.
It really does fit!! A truly simple bolt on clutch adaptation procedure. Well I'll have to lathe some of the original flywheel away because there is a simple recess (the bolts only have about two threads engaged at the moment) and also remove the redundant BMW ring gear but that's trivial.
The important thing to note is that the BMW clutch will then bolt into the additional flywheel. I cant do away with the original flywheel as the extra mass is needed to spin the diesel engine over compression. I will however be removing the weight of the BMW flywheel + clutch bits from the Kubota flywheel when its in the lathe.
So to finish the gearbox adaptor all I'll have to do is make a gearbox mounting flange and weld spacers in-between it and the convenient 6mm plate the starter motor is bolted to. At most a good weekends work only leaving fitting it into the frame which is a big but hardly a technical job.
Then whilst turning the crank to-and-fro looking at the two flywheels I became aware of a chonking sound. Then after some more thought and careful flywheel moving I concluded the big end (or little end) bearing of the rear piston was damaged as it allows about 1mm of flywheel rotation before the 'chonk' and the piston starting to move. 
Guess its scrap unless someone can convince me to continue and rebuild it (for cheap).
I think this will require more than a slug jamming into the gap.
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Kubota Z482 which is plodding on with unnerving reliability. Three years so far.
1900 Diesel Bike being rebuilt with better clutch control.
