Without being cryptic, there's some important differences between the top one and a Bellett pump from memory...we should play spot the difference! I don't think that one will work on an OHC Bellett, but I may be wrong.

Repair kits I've found have their perils...3 different types of seals within PR20 Bellett pumps (carbon, cork and steel sealing faces). Later pumps also have wider bearings, longer shafts and different impellors which operate at a different flow rates.

Not sure what the difference in bolts for backing plates means, but would be interested to find out. Sorry sdoubleb, didn't mean to scare you! The Cypress pump and the Bellett look very similar, yes, so probably good, but safest to check critical dimensions before buying just to make sure.

The impeller looks a tad large, but it's hard to tell from the photo, of course. You're right though, these are a similar design that will be on the market forever, so there'll always be supply if it can be adapted.

I think the critical dimensions would be the height of the shaft and size/design of the impeller. The impeller may (most likely) be removable and could be replaced with an original or repro part, but some may be sweated on.

Bill, who did my engine rebuild, had reams and reams of books showing critical dimensions of millions of parts, he seemed to have a way of accessing this information. He's closed up shop, however, but there must be a way to find out without having to purchase other pumps? At the time though, we found nothing similar, so there's no straight swap.