It certainly is possible to convert it over to a gas-powered unit. Doing this is actually pretty simple, once you have the Engine & various parts.
You won't need much:
1. Engine
2. Piloted/Unloader Valve: www.mastertoolrepair.com/pilot-unloader-...lves-c-25_3_682.html . * The "BG1" style Unloader would probably work for you, just choose your PSI settings for cut-in PSI and cut-out PSI. You'll just pipe in the small Bleeder Line in one of the ports and the Throttle Cable into the other 1/8" port.
3. Throttle/Idle Control Assy: www.mastertoolrepair.com/air-compressor-...ontrols-c-25_78.html . You can use a cable-style or a pneumatic-style that just uses a hollow, 1/4" nylon tube. Either way it'll install. Just choose the part by horsepower rating & length of cable (if using the bullwhip-style).

You obviously may have to change some plumbing fittings here & there, or drill a few holes for the engine, but that's simple stuff. Once you set the high-speed and idle RPM on the engine (once everything is hooked up) you should be off to the races!

Hopefully this gives you enough info to start. Reply if not.
Thanks!
- MTR

So, of course you would install the Pilot/Unloader Valve in the tank Inlet where the Pressure Switch is now. The Transfer Tube would remain in the main tank Inlet where it is now.
- MTR