I did the same on a m1650. Welded a section of 1/4" keyed shaft to the rear half of the original shaft, as the rear section still had great splines. A cast double groove pulley on the front. Member "Craig" has had the tractor for quite a few years now, and it's still working fine, and he uses the rear pto often.Sorry, to miss on your 1650 worn PTO splines. If your are talking about the upper PTO drive shaft. What you can use instead is 3/4 or 1 inch shafting with a 1/4 keyway . And for the sheaves, I used cast iron double groove. Works great on 2M MF GT'S 1650 with 3PT hitch and rear PTO.