I'm not sure on the size of the o-ring but it shouldnt be hard to find.
Pump has no moving parts, it stops pumping if it develops a leak that breaks vacuum, usually at the o-ring where it plugs into the left tank fuel level sender.
I developed the same problem and it turned out that the locking clip of the siphon pump was damaged when I replaced the fuel level sending unit previously. The siphon pump connection would work its way loose and stop pumping.
Fixing the locking tab solved the problem and I didnt have to change the pump.
Others have found the connections fine but have seen the o-rings hardened and brittle.
Worth a try to check first IMHO. Potentially a few cents vs $86.
