I wouldn't pull it just yet. I have seen much gunkier engines. If the compression measures good, the oil pressure is fine, and it makes no deep, clunky sounds under throttle, it is probably perfectly good. If it proves out, I would jack it up a bit, remove the oil pan, and wash the topside good w/ kerosine (diesel) or gasoline (don't smoke) until shiny, and wash all the carbon and crud down, clean the oil pickup screen. Force clean oil thru the passages using a small garden sprayer attached to the oil sender port (I do this on engines that sat for years) to wash any grit out of the rockers. Then bolt back w/ new gaskets.
