I've driven them for years and they are fairly durable engines, although sloooow from 0 to 60 mph. They do have a habit of getting cracks in the heads between the prechamber hole and valve seat, or otherwise out from the prechamber hole. A crack to the valve seat is bad, but other cracks are not fatal. The bottom ends are pretty rugged, though on manual transmissions the thrust surface on the crankshaft can wear and need machining. Not sure if oversize pistons are available yet, but standards are and the block can be sleeved at a machine shop (as it doesn't have sleeves from the factory) to bring it back to standard. You would want to check the condition of the prechambers for cracking or a damaged ball inside, as I have heard of and seen engines that have had the bottom of the prechamber break and fall on top of the piston. A rebuild requires a new timing chain, and a new kit for the vacuum pump is a good idea as well. New injector tips are probably a good idea too. Never heard of much of a problem with injection pumps. If these engines are maintained properly, they will run for many many miles without problems. Parts are still available, and I would get an itemized list for cost before you start, as the costs can add up quickly.

If you do want to retrofit to a new engine, I know a later 220D engine used from 1968 on will bolt right in, and I think the early 240D engines will too.