I haven’t done it myself but I think it was the late Art Sesselburg who did it and used the original dual fuel/air line so it appeared to be a functional tank. Up near the motor end connector, he cut the air hose and installed a small filter, something like a gas filter used in small outdoor equipment, to keep dirt and dust out of the tank. It looked good and no one could tell the difference with a casual glance.

Thinking about it, a person could go one step further and cut the air hose under the motors hood. That would really fool the masses!