You'll need at least an o-ring for a pump swap. Here are the parts I see in the EPC:

016 997 32 48 pump main o-ring
016 997 37 48 cut-off piston cover o-ring
126 277 08 55 cut-off piston big teflon seal
126 277 20 55 cut-off piston small teflon seal

NOTE - these part numbers are based on a difficult to read EPC screen.

I think you can get away with just 016 997 37 48; pop the cover and replace the seal. Let's see if I can find instructions so you know what to expect...

After reading page 50 of - http://www.w124performance.com/docs/mb/transmission/trans_722_repair.pdf - maybe you're right to install a tight secondary pump rather than address the leaking seal The inner o-ring is inboard of the the cut-off piston so a spring and the piston have to come out. Easy enough but installing the piston requires compressing the teflon seals as you would piston rings. More steps in uncharted territory opens the door to mistakes

Sixto
87 300D