I agree with the other posts that say a 20 year old diesel engine is not anywhere near end of life, IF maintained. Change oil, filters, coolant, and check/replace rubber parts as needed or scheduled by Yanmar.
Diesels need to work hard, so if properly propped you should be running it at least 80-90% of rated RPM. Your Yanmar should have the rated “continuous speed” stamped on the engine plate or in the manual. You should try to run at that speed (or close to it) because that is what it was designed for. If you run it too slow it will form carbon deposits in the cylinders and exhaust elbow, leading to shorter life. (I went to Mack Boring, Yanmar’s US distributor, and took a 2 day hands-on owners-training course taught by the guy who trains the dealer mechanics. He put a lot of emphasis on this.)
PS - Mack Boring is located in Somerset NJ and I highly recommend this course. They do it once or twice per year and they bring in the same or similar engine model you own into the classroom so you can wrench on it, with guidance from the instructor. They teach diesel theory of operation and hands-on maintenance including valve adjustment, major parts replacement etc. You will come away with a much greater understanding of your engine and gain confidence, and that knowledge will also enable you to interact more productively with a mechanic when you need one.
PPS - A cheap $1k Chinese outboard auxiliary that’s rarely or never used, fueled by (stale ethanol) gas, does not sound like a reliable or viable backup plan. Keep your anchor ready to deploy.