Life expectancy is largely a function of how it has been maintained and how it’s been run and there is no real way for you to tell that short of availability of a
maintenance log and a conversation with the previous owner.
I have a mid-eighties Universal M50 with slightly fewer hours. I expect it to last a good long time, with proper care. It has good
compression, starts instantly, and burns maybe half a quart of
oil every 100 hours of operation. It would not be unreasonable to get 10k hours or more out of it.
Universal used to be an independent company before it was bought by
Westerbeke. They use Kabota tractor engine blocks and are pretty robust. Non-marinized
parts are available at Napa.
If your really concerned I would get an engine
survey done. If it comes back good you have some peace of mind as well as suggestions about minor issues that might need attention. If it comes back bad you can decide if it’s a show stopper.
Some of the marinized
parts are getting expensive and/or difficult to find. An engine
survey should be read with that in mind. As a freshwater
boat that should be less of a potential issue.
Replacement would run you $15,000 (new
motor and labor) and up, dependent on how difficult
removal and
installation is on that particular
boat and how all the related
gear (exhaust, wires,
fuel hoses, etc) are.