I wouldn't underestimate the power of the one lung Kohler K series engines. I do, however, understand your power needs may be a bit greater than these engines can deliver, even the 16 horsepower, which, I understand, are rare.
I concur with Madhatter on the GM Delcotron alternator. There are several styles of that one wire alternator available, and you should look for one with a built-in voltage regulator. I think they were popular in the 1980's cars and trucks. Look for a large ampere capacity. As far as a compressor, it seems that from your posts, that you want a compressor to power an air horn. If that is the case, I would recommend a Bendix air compressor from a medium duty truck that was air brake equipped. These are fairly easy to find in a truck salvage yard, and they will have air reservoir tanks and a place to mount the air governor on the compressor, making it easy to plumb. Problem is with installation, your engine will need a full oil pressure system to provide for the lubrication of the air compressor through the engine oil pressure system. You will also need an air pressure and reservoir gauge, but that is a minor problem to install. Lubrication is a problem with compressors, because most don't have their own lube system. You will find this problem with air conditioning compressors, such as the York, as suggested previously. Perhaps considering an electric air-sounding horn is a better alternative. Just a thought....
I hope you get this project together...It sounds like a fun adventure.
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