The enormous standing Army in Australia seems to have been in place for a relativly short period whilst Japan was threatening to invade.
Its possible that the M20s in Australia visited the workshops for repairs just as much as anywhere else but because the workshops only had one or a few machines in at any time they were fixed and returned to their units with original engines, the mass engine changes in the UK, Germany and other places are more likely to be post war or in workshops full of machines where a production line system was used.
We did the same as every other army
When machines came in for major service it was swap the engine / gearbox then back out ASAP
When they sold off the WDB40's about 1/4 of them had mismatched engine & frame numbers which became a problem latter on when we tried to get them registered
Some where I have a copy of the order for B40s.
Went some thing like
495 bikes ( 500 including the 5 test bikes ) + 40 engines + 20 complete frames + 20 complete front ends
The rest of the spares never made it as they got sunk in the Suez canal
