If one were really intent on doing this, contact Superior Diesel in Rhinelander, WI. they are a John Deere engine distributor. In the late 80-early 90's, they had repowered 3020s (4239 etc) and 4020's (6414) repowered tractors at some farm shows.
At one point, they made a kit, or at least how-to instructions, to install these engines in some of the NG tractors. They have maybe long since forgotten that they did, but they were out there!