Here is a very nice and small Brunner Air Compressor. Many people have used these to make model gas engines. It seems to be in pretty good shape but I don’t think its ready to make any air, It was tight, I loosened the main bolys and sprayed with WD40 and it now turns but tight. It has been sitting a long time and will need cleaning. Check out the pictures and see if you need it. The bore is about 1-7/8 in diameter but it is difficult to measure so that might not be perfectly accurate. The stroke is about 2. The crankshaft is about 15/16 in diameter. No cracks, breaks, or welds that I can see or know of.