For the cost of rebuilding or less you can repower with a brand new engine that will be cheaper to operate and maintain overall.
With all due respect, garbage.
The parts to COMPLETELY rebuild a single cylinder diesel engine, with the exception of the block and head, even a Volvo, won't run more than $1500. Provided the block is decent, the head can be decked, cylinders honed, bearings replaced,... in fact if you can fix a lawnmower engine, and you're not afraid to get greasy, you can rebuild a single cylinder diesel engine in a weekend, if you have the parts on hand. It ain't rocket surgery. And, of course, this is assuming that the engine NEEDS to be rebuilt. Jones's Diesel rule of thumb #32(a) if it starts and runs, it usually only needs maintenance, not a complete rebuild.
Oh wait, you must have meant "the cost of HAVING SOMEBODY ELSE rebuilding".... and even then, I call bullschlitz. take the engine to the local tractor-and-forklift shop, not a "marine mechanic" and come back a week later. A shop would be hard pressed to invest more than 20 hours of labour into a full rebuild of a one lung diesel. even at $75/hr, plus $1500 in parts, it is still less than the cost of a Beta.
And let's not forget the hidden savings- the old engine goes right back in, whereas a new engine requires new mounts, new fabrication, headscratching, maybe a new shaft, etc., etc.,
I like the new Beta engines, and if i was commissioning a new boat, building a new boat, had a $8k-$10k budget for a repower, or had no mechanical skills, no tools and no desire to learn, i'd consider it.
But there is no way it is cheaper than rebuilding what is already there.