Morning Neil
Sorry I have had little input to your posts, my attention has been directed elsewhere unfortunately.
Let me try to answer your latest questions.
Leaving the 250 & 350cc machines to one side. What You have in your latest pictures, is one of each, of the three 5/600cc Aero engine types produced, do you have the flywheel side, this has the engine No stamp on it.
Picture 1 with the cast magneto stand, this is the correct casing for your 1937 machine, this crankcase type would be prefixed 6P for 1937 and 6Q for 1938
Picture 2 this engine type dates to the later 1936 Aero period, it has a separate removable magneto stand, and was the first of the longer crankshaft models it only came in the 600cc variant and the engine No was prefixed 6M.
In your picture there are two holes drilled in the casing at the back of the main bearing housing, this is another bodge someone has done to allow them to punch the bearing out.
Picture 3 this is the early 1936 engine type and was used for both the 500cc units and the 600cc units and would be prefixed 5L or 6L, again with the removable magneto platform, however this crankcase with the crescent moon shaped oilway at the back of the main bearing
accommodates the shorter crankshaft type. The shaped oilway allowed more oil vapour to the additional components of the shorter crankshaft ie the extra parts for the oil pump drive and the Dynamo drive.
Your comments around the crankshaft fit.ie it’s loose within the bearing.
The two main bearings are shrink fits into the crankcases housing and therefore tight on the outer race, assembly of the unit is allowed by the crankshaft being a push/sliding fit into the inner race. Crankshaft end float is set with thrust shims on the flywheel end of the crankshaft and with the correct size of gasket between the crankcase half’s, look for 6thou of an inch end float when fully assembled.
The damage to your crank case looks like some buffoon has attempted to take the crankshaft out without removing the oil pump drive and timing gears as there is still a slight step at the back of the bearing, ie it’s all gone inwards rather than outwards.
I agree with Eddy, the way to fix would be to let in a thin ally washer shaped piece.
Note that all the crank cases are all matched pairs, stamped on the case under the front cylinder so if you do go along the line of mix and match you may have to do a little alignment work.
The timing chest main bearing is SKF 6105 with the dimensions you stated with a CH3 clearance and 05 to the outer race.
Steve