Any low volume 12v. automotive fuel pump
will be a good substitute. Simply specify that the pump has EPDM or EPR rubber components.
Just put the automotive pump in series somewhere on the fuel delivery
line .... and unless the mechanical lift pump is leaking externally, simply leave it in place as the flow from the automotive pump will pass right through.
This will get your friend 'up and running' and will allow more time to find the correct pump.
Once you find the correct pump, leave the automotive 12v pump in place and use it to pressurize/bleed the system - all you need is a momentary 'push button' to energize.