The engine is already out, as I wanted to check / replace the aging diesel tank, and clean up / repaint the engine compartment.
As the engine did work OK in general, and started immediately each time, it will probably last a few more years. My main comcern is putting cash into rebuilding (if necessary) a block that has been raw (salt) water cooled for 25 years.
My brother can:
a: Just clean it up, so I can put it in the freshly painted engine compartment.
b: try to do a compression test, although he will have to borrow the tools, ask someone to help, as his auto engine compression tools won't fit the diesel injector holes.
c: take an oil sample in for analysis, but it has been sitting for a couple months - would the oil sample be a true picture, as particles may settle out.
d: remove the head, so he can see the piston/ ring tops, valves, seals, and maybe see more of the block water jacket, to see if there are major corrosion issues.
If he can check some of the above, and put it back together for a couple hundred bucks ( I need new engine mounts, approx $200 as well), then I will put it back in.
However, I think that if anything major need replacing, I would opt t oput my cash into a new engine, instead of one that old.
I would also like to know what the normal changes would be, in going form a 13hp engine, to a 21hp new one. Would that normally require a larger prop, different pitch, etc.
The 3YM20 is a couple inches longer, but I have room to push the shaft / prop towards the stern a couple inches. As well, I know a retired prop guy, who could probably shorten / rekey the shaft, if necessary.