I seem to remember seeing an article about repowering Deere choppers with bigger Cummins engines in Farmshow a year or two ago. Maybe Farmshow can find the article or tell you who did it.
I would talk to Kinze Power Products in Iowa. Their website is kinzerepower.com. They repower the JD articulates with 855 Cummins Engines. I have seen a converted 8850 and it made a great tractor out of it.
Stumbled across the Farmshow article about the repowered Deere chopper. It was a 5830 repowered with a Cummins 855 out of a truck by GRK Enterprises in Fairview OK. Their phone number is 1-405-227-2903 HTH