Meant to reply to this before, even got round to registering to do so, but got distracted.
I used to have a Thornycroft engine from 1988 in my boat, until it decided that internal combustion wasn't a game it wanted to play anymore.
Mine was a Type 80D, which I found out, when mine died, can suffer from cracked pistons.
There is also the Type 80E which was around at the same time which has a number of different parts including the pistons so maybe that was a development of the design which overcame the issue.
You should be able to identify which engine is fitted to the boat you're interested in easily as there should be an ID plate on the flywheel housing.
I have no idea how many hours were on my engine when it failed as there was no hour meter, and I suspect that it had been somewhat neglected by the PO.
In tems of parts availability, the usual service items are fairly standard and available from chandlers, and it would have been easy to get hold of the parts for a rebuild had I not decided to fit a new engine, supplied by Thornycroft with the bits necessary to attach my existing marinisation parts.
Hope this info is useful to you.
Andy