I agree with mkirsch. A 9 speed M doesn't sell for very much more than a regular M.
You might also consider selling your M to buy a newer tractor with more usable speeds, live PTO, live hydraulics, fenders and more comfort for the same or less cost than rebuilding your M to 9 speeds.
IH 300 - 560, 706
Oliver 77- 880
Case 400 - 830