Here's a place you might want to try -
Then and Now Automotive Phone 781-335-8860 and webpage
http://www.maritimedragracing.com/then_and_now_automotive.htm They sell rebuild kits for the Henry J fuel pumps. I don't know if the Darrin is the same. The Henry J originally used a Carter pump (on both single action and vacuum boosted versions). Kits for those are about $60.00. They also rebuild them if you send yours to them.
I really do not like the AC pumps that many of the club vendors sell. While they work, you have to replumb the hoses/tubes, the AC pumps are noisy and they spew oil out the side all the time. Therefore, I avoid them when I can. Give this place a call and see what they say.