Have access to a new old oil pump for 851 so wonder if works on 1990 900ss engine ....same flow?

it`s the same (#25010021A)

25010021A 851 on steins page

25010021A 900ss on steins page

Agreed but the 851 oil pump I looked at was 24920011A

Same unit but different number?

I don`t certainly know but it looks to be that way.
Just looking at Steins page:
24920011A (which is unavalable) is the replacement for 066770200 of the P750, 750F1 & 851SP '90

If you look at the P750 parts cataolgue it says: 25010011A (as replacement for 066770200) and the 851SP '90 parts catalogue says: 25010021A (like 900 M/SS).
Imho this means they are all the same and interchangeable.

Thanks heaps .... Makes sense