The average lifespan of a good marine diesel engine that is well maintained should be around 10 k hours. The average life of a regular gas engine is around 2,500 hours, so the Atomic 4's that are still running have usually already been through at least one rebuild.
If /when your engine fails to start from low compression because of worn piston rings, etc. then you should always rebuild an engine first. "Repowering" refers to complete engine replacement, which is only necessary if there is catastrophic failure of the engine through flooding, serious overheating or fire damage, etc. Marine sailboat engines usually die from lack of regular use and corrosion through neglect, not from being over used. The Universal mx25 is an excellent engine that was the best motor that Catalina ever used for their auxillary power. We use the tiny 5411 diesel in our 1980 C 30 that has already been rebuilt at least once & is kicking around just fine. If U decide to repower your engine before her time, then ship it to me. I'd be glad to put another 8,000 hours of run time on her!