I cant offer much advice mechanically, but I can recomend you test in the water. I looked at a motor earlier this year, it looked imaculate, ran perfectly on muffs. I put it in the water, idled out of the channel lovely then I gave it some stick, after 2 mins over temp alarm started. Outcome was a new head and thermostat total just under $1000. I was lucky enough to find this out before purchase.
So if you can check any motor under load.