every time this comes up, i think of the water pumps used on a zillion Holdens with the Grey or maybe the red motor (or black) and think those simple pumps might just be adaptable to a Bellett. Mind you I don't have either of them to compare and see if that's feasible.
Depending on the height of the Holden pump, an aluminium adapter plate might work ? eg a plate 10mm thick, drilled to Bellett holes and countersunk bolts used to screw to Bellett block, then tap the plate to take the Holden pump. All assuming the water passage would line up or close enough.
You might be onto something Glenn...I wonder if it might be possible to find a proprietary pump shaft/bearing/impeller of the right length (maybe Holden), scan in the Bellett water pump housing, CAD up a modified housing to accept the new shaft/bearing/impeller and then CNC machine a new housing? Off shelf aluminium might do the trick, a coarse sandblast on the outside will make it look like a cast housing...completely rough estimate, maybe $100? Add backing plate, gasket and proprietary parts, maybe another $100? Maybe we can 'open source' our way out of trouble and have a reliable supply of parts?
I wish i had a bellett and holden pump to compare..... i am sure there is something that will work. More likely to be an older model car of course with simple pump designs.
The simplest way might be to get a pattern made and have some sand cast housings made.....with your idea of using an off the shelf shaft and impeller. I imagine the cost of a pattern would be fairly low as its not a complex design. Or if a good pump housing was available a good caster could make more of them using it as the pattern although casting shrinkage would need to be taken into account. Even a 'bad' sample could be bought up to 'new' using bog filler to make a pattern.
But all this will cost money and would need a number to be made. And also it implies that the shaft/impeller etc is available also.
I like the idea best though of finding a suitable pump from another car. Best done by a visit to an understanding Parts place that has various pumps in stock you can compare with.
CNC maching of course will make whatever one wants, but i suspect the cost of programming will be very high.