I drove an 82 D50 for many years long ago. The engine was decent but was worn out at 175,000 miles on it.
Don't know if it will help you or not, but I do recall that the engine was the same as the earlier 4 cyl Caravans had in them. I know because I used the valve train off of a Caravan to repair mine. I think the head casting is different - maybe it was the distributor location (end vs side) that was different. Not sure if the block mounts would be the same - I suspect probably not. In any case, the engine internals I suspect would be identical. If you can find one from a Caravan you could maybe use parts to fix the current engine. Probably a LOT more Caravans were made with this engine than there were D50's...
Another option is to repair the current engine. I put bearings, wrist pins, and rings in mine. Not all that difficult to do and I'm certainly no ace mechanic. I did not have an engine stand so I did the whole job with the block just laying sideways on my workbench. It is not too heavy and you can jockey it around as much as is needed to get this job done.
-Deon