Check the tractor salvage places, they might can find a used engine that still has life.
That's very labor intensive job which he wouldn't be saving money going that route. That john deer is a wet liner engine and should be able to do an inframe in it if the cranks good and doesn't need turned or polished etc. If its inframe and places beside a dealer that job would be any where from $500-1000 in parts depending if you went with john deere parts or aftermarket imported parts. Labor will vary from shop to shop since shops hourly rate varies.
