Mike,
Nice score on the $100 129. I got a $150 one off of Ebay and it was a complete wreck, though after much work I made it respectable again. The good trunion bodes well for the condition of the rest of the tractor.
As far as the head goes, well, you know why the engine failed, and with the head off you can probably make a good guess as to the condition of much of the rest of the engine. If the plug is clean and there isn't oil on top of the piston you are probably good to go once you get a new head.
As someone who has mixed and matched Kohler parts over the years, here are a few tips:
Try to find a head that matches the configuration of the current head, with regards to the height of the bolt holes and spark plug location, or else you will be hunting down spacers, buying new head bolts, and cutting sheet metal to make it work. New bolts aren't a bad idea either. Check the spark plug hole as well, many used heads out there have stripped out threads or have inserts installed in them. Take my word for it, those inserts seem to stick to the plugs rather than the head, even if you Loctite them in. Finally, make sure the new head is flat. A thick piece of glass, a piece of fine grit wet or dry sandpaper, and some elbow grease will take care of minor warpage, more serious cases should go to a machine shop.
