Hi Dave

Welcome to Mankydom. Not sure if you can buy the diaphragms seperately. You can certainly buy new fuel pumps. I always recommend Michaels of Selby, (their details are in the Links for Commercial Purposes section of the forum. Very helpful & knowledgable guys & fast mail order -tell 'em we sent you.

). I bought a new pump from them a couple of years ago. Think it was about 30 quid.
I blanked the mechanical pump on my red Special Delivery Reliant trike
red trike& fitted a "Facet" electric one. Worked O.K, but was high flow & tended to try to force feed the carb, leading to leaks all over the place. It also kept sticking, meaning I'd have to pull over & give it a whack wiv a 'ammer to free it again.
Some guys have tried removing the pump altogether & relying on gravity feed. I think Triker Pete did. If I remember right his Reliant used to run out of puff on steep hills or under load though.
A good condition mechanical pump is probably as good as anything.