Ran across this thread on google, just figured I'd throw in my .02 from exp with the westerbeke 33. It should be rebuildable, we had ours rebuilt about 2 years ago, due to 2 cracked pistons. Almost no engine is un-rebuildable, the question is how much are you willing to spend to rebuild it. If yours has never been rebuilt, and it only cracked a piston and the crank/rods/etc are in good shape, and the cylinder isn't torn up (probably isn't), it shouldn't be too big of a deal to rebuild it at a reputable rebuilder who's capable of dealing with diesels. Cylinder overboring isn't a big deal as long as pistons are available and they are. The 33 tech manual lists .025, .050, and .075 overbore pistons. The greatest expense with rebuilding the 33 is that you generally have to get all the parts from westerbeke, which is very expensive. I would suggest taking the engine to an engine rebuilder and find out what they would want to replace, and what they'd charge. You should be able to get parts numbers and prices from your westerbeke dealer. From what I understand it's pretty much impossible to get a discount on parts.
The only other thing to be concerned with when rebuilding is if the cracked pistons are the only thing wrong with it. If they are, great, but if you have more serious issues (in particular with the injection pump, which as I recall wasn't rebuildable, or possibly was for $3000 or something ridiculous, we were going to replace the engine if the injection pump had gone bad).
The 33 is a mitsubishi built diesel, but we've never been able to track down what mitsubishi model it is. The only way we found it was mitsubishi was the new pistons from westerbeke have the mitsubishi logo stamped on the underside. There were no mitsu part numbers to be found anywhere. On the plus side, the new pistons were redesigned to improve combustion and made an incredible difference in starting the engine (although not having cracks in the pistons makes a big difference as well