I'd like to update this information to add that Moyer Marine now offers an "M-502" pump that is a direct, bolt-on replacement for the Oberdorfer 202M3, M7 and N202M7 pumps.
It uses the exact same impeller as the above listed Oberdorfer pumps and uses packless ball bearings for the impeller shaft, eliminating the grease cup fitting and is permanently lubricated. The cover plate uses captive, knurled thumbscrews and a permanent O-ring, eliminating the need for paper gaskets. This makes swapping impellers a stress-free, 30 second affair. Lastly, an adapter is sold which will convert M30's with the N202M-15 over to the 2-bolt mount. The M-502 pump also uses the same 3/8 inch NPT fittings as the Oberdorfer pumps.
I have no professional relationship with Moyer Marine, just a very satisfied customer.
Here's a direct link to purchase the pump: http://www.moyermarine.com/cgi-bin/shopper.cgi?search=action&category=CSOB&keywords=all&template=Templates/B000_storebuilder.html