Two things I've seen that are hard to get around replacing an engine in a Kubota tractor. Most use the engine as the frame to attach the front axle, radiator, etc. There also has to be the hydraulic pump drive at the injection pump.
The plate that bolts to the engine for the bellhousing is the same bolt pattern from a Z751 to a V2203. The center hole is bigger for the rear main seal cover on the V1903 and V2203. The V1501 in my L345 had the same measurements on length and height as the V1903/2203. It had the smaller rear main seal cover.
How do I know all of this? The L345 was so worn out when I bought it, you had to tow it to start it. It had more blowby than exhaust when cranking it with the starter. I bought a V2203 parts engine to experiment with. If the head wasn't cracked I would have used it in the tractor. I bought a V1903 in Pa. on Ebay. I picked it up on a trip to Upstate N.Y. It didn't have the holes to mount the front axle on the tractor.
I had a feeling I needed at least one piston for the V1501. It needed two. I have a Z751 I got for $50. It is 1/2 of the V1501. I used the pistons, bought some Kubota gold plated rings for $307 and fixed the V1501.
From what I have seen searching, the V2203 is more available. If it has the holes for the front axle and hydraulic pump drive, it should be a bolt in replacement. I don't know if direct injection/indirect injection makes a difference, without glow plugs or with.
There are different versions of these engines.
Good luck.