Well, to test the operation it wil be necessary to invest in a motor and belt. If the unit will build pressure and the blowby is moderate (.5 cfm @ 3000) then the problems begin to revolve around cosmetics. Check for leaks, and other obvious stuff. The corroded tubing is a PITA, use strippers to clean the fins and follow up with paint. I wonder if the cylinders/heads could be blasted with walnut shells or something?
I've done restorations of small, MIL surplus compressors but I had the benefit of miscellaneous parts and partial blocks which I could strip. The suplus compressors had been beat up by surplus dealers, thrown around and neglected. One was covered with dirt and grease and looked like crap. I blasted it with ENSOLV which cleaned it up enough so I could see how to work on it. Disassembly showed almost new internals, very nice. When finished, after straightening fins, painting and all the mechanical work and cosmetics, the compressors looked and functioned almost like new. They will really jam tanks.
I've actually got a 5 horse Honda engine, sheave, belt and bolts to take with me. I'll drain the oil, see what that looks like, refill it and fire it up to see what happens. I've read some of your posts on how to check for blow-by and drop rate for third stage piston. Is there any way to check con rod and crank bearings? Or should I just rely on overall smoothness of running? If bearings are shot, I'm guessing it will run rough, noisy and vibrate like an engine with spun bearings. You mention blasting the block with walnut shells...the owner also mentioned that. He is a retiring dive shop owner. Is that using a bead blaster or sand blaster with ground up walnut shells? I was already thinking of bead blasting it after I sealed everything up good. For the frame, I was going to start out with stripper and then sand blast it, replace the sheet metal and repaint it.