I would definitely not replace a wartime engine for a post war one. The worth of your M20 will drop instantly. I would put all effort in restoring/ an overhaul of the original wartime engine. Maybe you could use some of the ZM20 engine parts for the restoration of the wartime engine?
Yes, I agree with Bastian...All you need is a wartime set of crankcases (WM20 prefix), a timing cover and tappet cover and you can build your ZM parts into that to make a WD engine..
If your front wheel is as original, tap the spindle out from the offside, ie to come out from the brake side. There's a shoulder on the spindle that the brakeside bearing inner sits against. Take a look at the Draganfly site: there's an exploded diagram that shows the hub assembly.
If it's all seized up, it may pay to soak it in parafin for a few days first...
