The fuel pumps can be removed on the M-18/M-20's by removing the small access cover.
I have done it multiple times, just be a bit careful as mentioned.
No need to go hacking an access hole in the cooling shrouds.
The newer plastic pumps use a rubber sealing ring instead of a gasket making it easier to replace without gluing a paper gasket to the pump.
Do read the instruction paper provided with the new fuel pump, as they are not tightened as tight as the metal pumps are using a paper gasket.