I have a 5U d2 and was wondering if a pony motor from a 3J D2 would fit? If not what would have to be changed. Thanks for any help.

The pony motor top cover is different on the U series than the J series, mainly the water outlet to the diesel cylinder head is on the side of the U series covers and in the center of the J series. The covers will interchange without trouble. The exhaust pipe will probably be different between the two, but both will fit the different covers. Probably some differences in the breather/oil fill attachments as well. The blocks and everything in them should be the same.