I have a Westerbeke W-52 in the same general design ballpark as the 4-108, but I have the opposite problem: too few hours. There's only 1,260 hrs on this 18-year-old engine, which is what happens when you build a motorsailer on the Great Lakes and then never leave the Great Lakes: 80 hours a season of use.
We intend to leave the Great Lakes on a circunavigation in '09. My mechanic is suggesting at the very least a top rebuild (visible condensation rust on the valve springs, rockers, etc.) and I will haul the engine for this next fall. I will also get the usual oil analysis, inspection/flushing of heat exchange, oil and transmission coolers and other bits and pieces. Add to that a full paint job, gasket change-out, belt(s) replacement, replacement of all original hoses (none are rotten, but all can be spares), and the change to a Balmar 100 amp alternator and I will be far more confident that this heavy chunk of change will be good for the 2,000-3,000 hours we could easily put on her in a five-year trip.
If the engine isn't worth rebuilding (and there is no real sign that it isn't), then I will go for something similarly powered as I am also getting a custom VariProp that will be made for the power bands this model achieves.
So my points are two: a) not enough engine hours are possibly even more destructive than too much, and b) as far as I can tell, a "prophylactic" rebuild with an eye to keeping the better cast-off parts as spares (depending on your carrying capacity) is an exceptionally good idea. Even a full rebuild that replaces the entire drive train in the block is going to be significantly cheaper than a new equivalent diesel, and you won't have the issues of redoing motor mounts, couplings, etc.
Not to mention that while the engine's out is a great time to clean up the whole area, replace the engine mounts, move to PSS-style stuffing box if you like and to pull the shaft for inspection for trueness and scoring. Also, you can refresh your exhaust hoses, waterlift, fuel filtering system, etc. If they are of the same vintage as the engine, they are probably due for replacement. Bad fuel and back-pressured water are the two worst diesel enemies in my admittedly limited experience.
It may not *seem* like an opportunity, but it will raise your confidence considerably while on passage.