Hi, 9000 hours is quite a few, my friend has a VW Passat with 300000 miles on it, and even averaging 40 mph that would equate to only 7500 hours, and we wonder if the car will last another week! ..... mind you, we have been wondering of its life span for quite a few years now, some engines just refuse to die, other bits of the car seem to accept death more easily.
No doubt being a 'rental' it will have had plenty of oil changes and might last quite a few more seasons just like the Passat, but as you have pointed out, as with all machines it will one day die.
If it were my situation, I would look at the engine lay out with care if considering a different engine, as the complexity of alignment to the shaft, and squeezing in a slightly bigger engine into the available space can be quite a headache, not to mention the extra costs. I would budget for a replacement engine like for like, 56 HP isn't too bad, negotiating on the price to include the hassle factor, and then keep the old engine going until it becomes unreliable or dies, it could be years of happy sailing. Other considerations must include new standing and running rig , and probably new sails, and possibly new instrumentation .... all of which add up quite quickly. With consideration to being an ex rental, a re engine is a relatively simple fix, the condition of the internal finish if beaten up will be more difficult to reinstate, but that is just a personal thing of what condition you can live with .... it will be a pleasure to sail in any condition I'm sure.
It is really just your evaluation of money you spend v what you want, and the decision will be yours alone.
Sorry not to have vast knowledge of the exact model, sure some experienced folk will comment soon with things to lookout for on this model, but thought to give a little food for thought whilst you are waiting, lots of well informed people here.