i tried using a lawn mower engine to run an automotive alternator for battery charging. It worked ok, but was really noisy. It would take a good size engine to run an a/c compresor, i would bet at least 5 hp if not more. I dont' know much about cooling systems, but i do know that freon or the modern equivilant is not cheap. Window a/c units are super cheap and easy to find in the newspaper want adds. I would think a couple of these would be a better starting place than trying to assemble an auto style system run off of a gasoline motor.
one quick note. Auto a/c compressors are some of the most impressive air compressors i've ever seen! The freon had all leaked out of the a/c system on one of my old cars, so i converted the compressor to compress air and fill an auxillary tank. With the engine at 4000 rpm's, that little compressor would easily out-perform the 220volt 7hp commercial air compressor i have in my barn.
Quis custodiet ipsos custodes (who will watch the watchmen?)