Agreed, it's a bit of a gamble. For local cruising, I wouldn't mind a rebuild, as I'm rarely more than a week away from a critical part.
When you swap the engine, the costs are much higher (2-3 x higher than rebuild), but you get everything refreshed and reliable (electrical, pumps, sensors, seals, bearings, engine mounts, fuel/exhaust lines, wire harness, maybe new fuel tank, thru hull and strainer, fuel filter, prop, shaft and cutlass?, etc...).
So I would agree based on the OP's extensive cruising plans, from a financial point of view, keep it on life support until it dies or he's ready to replace. Make sure you have towing insurance!

I've seen diesels runs for hundreds of hours with noises / bad compression. Yes it will likely get worst, but it may also keep putting along...
I'm guessing you're looking at the Beta 35? Dimensions are nearly identical to your existing Universal. Same engine mount width, shaft center to engine bed height should be withing 0.5" if I recall correctly. I can't recall if the exhaust is on the same side or not, but beta does offer a crossover elbow option, which we've used in many installations.
The servicing of the Beta is very easy, as everything is up front. The built-in oil change pump is worth it. They aren't quite as smooth/quiet as a Yanmar, but that can be mostly compensated for with modern engine compartment sound abatement materials. For long distance cruising, I would opt for a mechanical (ie: non electronic) controls. Even my local dockside mechanic that specializes on smaller boats doesn't have the software / subscriptions to work on the newer electronic diesels.
If that's what you're thinking, I'd get it sooner rather than later. Who knows when emissions requirements may change and may render the mechanical engines un-admissible for sale in NA (probably not during this administration?)