I'm in Digest mode on the List, but I saw some posts about low oil
pressure in an aircooled van, replacing the oil pump with the engine
installed, and the difficulty in finding a replacement pump.

I wouldn't suspect the oil pump until all other causes have been
exhausted. If you replaced your bearings and are SURE that you have the
correct clearances there, your sender might be suspect. These can easily
go bad, and many times they also leak. You can either replace it with
new one (they're only 5 bucks) or install a mechanical oil pressure
gauge and see your actual oil pressure.

The VW Type 4 engine used basically the same oil pump from 1968-1983.
It's an aluminum unit with steel gears (021-115-107). If you are going
to try to remove it, you must be careful not to break the mounting ears
off by prying on them. It helps to first loosen the case bolts above and
below the oil pump, as the pump housing is a press-fit in the opening
when the case halves are bolted together.

If a new stock replacement pump can't be found economically, you can buy
just the gears from the dealer. However, this won't address the circular
wear pattern in the body or end cap, so it's only a partial fix if your
pump is truly worn out.

Many vendors sell a "heavy duty" version of the pump for around $60
(Melling is one manufacturer). It's basically a cast iron,
higher-capacity version of the original. It features a longer set of
gears that can pump a higher volume of oil. However, it may be necessary
to grind down the pump drive tab slightly so that it doesn't interfere
(bind) with the cam gear (the heavy duty pump extends a bit further into
the engine than the stock aluminum unit.) The drive tab needs to engage
the slot in the cam gear but should not be pressing hard against the cam
gear when fully tightened down. This causes undue wear on the cam thrust
bearing.

I have used the heavy duty pump on VW vanagon and Porsche 914 engines
with no problems whatsoever and would recommend it highly.
Alternatively, I have reused the stock aluminum pump after measuring the
clearances (I think this info is included in the Haynes manual), again
with no problem.

Hope this contributes to solving your problem.

Dan