I fitted a modified A/c compressor to mine, and had a gas bottle receiver in the back of the van. Used a standard compressor switch to switch the electro magnetic clutch in and out on the pulley.
Mine was never mega successful sadly, I had great difficulty keeping the oil separate from the air, inside the compressor. Different types of a/c pump might lend themselves better than the one I used, plus I only had hand tools back then. Its a great solution though, very inobtrusive, quiet, you can speed up the engine for more air, and those pumps move alot more air than you would expect. Mine had 12 pistons 1" ish diameter 1" ish stroke.
Hi, Air con compressors will work but not the rotary type, older style 'York' compressor would be better as they're more conventional crankshaft and pistons. The rotary type pistons are arranged around the shaft and operated by a skewed disc on the shaft and need the oil in the system for lubrication. The York ones are heavier and take quite a bit of power and are found on older car systems. Buses and coaches have conventional piston compressors but are bigger and heavier still.