Hook up a fuel line (gasoline) from the carb to a pump, to a length of hose with a quick-connect on the end (such as Evinrude) then find the wire that goes from the controller to the propane solenoid.
Install a SPDT switch in this line. Panel to common, other end of wire to one contact of switch and second contact to carb shutoff solenoid and the to electric fuel pump. If you see the power lead goes to a vacuum switch on the intake manifold as well as the LPG solenoid, make sure to install the DPST switch between this and the controller. This is known as a vacuum-proofing control for the propane setup and can be a real PIA.
This setup would give you the best of both worlds as you could switch from one fuel to the other simply by plugging into a portable fuel tank and flipping a switch.