While you are searching for a mechanical pump, you can pick up a universal electric one at any auto parts store very cheap and mount it on the firewall or inner fender. Hook the black wire to a good ground and hook the red wire to a source which is hot when the ignition is on. Disconnect the fuel lines from the mechanical pump (but DO NOT remove the mechanical pump from the engine) and (with some extra fuel line) attach them to the new electric pump. There should be an arrow on the pump and the line coming from the tank goes at the rear of the arrow, the line from the carb goes to the front of the arrow.
Although electric pumps are best suited to pushing fuel from near the tank rather than pulling fuel from the front of the rig, this fix WILL work and will provide you with a safe, functioning fuel delivery system until you are able to replace the mechanical pump.