While non of this is real helpful to JPS's original question. I would have to agree with you about John Deere and all there bs serial numbers Mumbo jumbo. Just like a 404 out of a 105 will bolt right into a 4020. I'm sure no different on the 00 and 20 series combines as well. I'm sure the serial numbers are different but the only real difference is combine oil filter is located externally instead of internally, and swapping out the manifolds and oil pan. Wouldn't think it would be any different today. Although all the electronics they have now may be an issue. If you can't get any answers on here and you have half way decent memory. Go to a salvage yard and look over a 12.5 from a tractor to see whether all the bore holes are in the casting that would match up to your combine mounts. Then I would think it would be just matter of changing out turbo, manifolds, oil pan etc if needed. But then like I said before wiring/electronics could be the deal breaker on on today's machines. Imo John Deere and every other manufacturer grabbed every farmer by the balls when they eliminated solid connections to drive components and replaced them with electronic componants. Nice and easy to operate I'm sure, but a PITA when it doesen't.