Are you good with a welder? anything is possible!
an early Cub Cadet 3 speed with creeper would be about the easiest swap because everything is inline... but a peerless 2300 4 speed would be my choice... thats what was in the manual shift tractors similar to your 1650 but it wont just bolt in because the rear section of the frame is differant than your hydro frame.
but really the simplest option would be to find an MTD 860 or 960 to fix up.
an early Cub Cadet 3 speed with creeper would be about the easiest swap because everything is inline... but a peerless 2300 4 speed would be my choice... thats what was in the manual shift tractors similar to your 1650 but it wont just bolt in because the rear section of the frame is differant than your hydro frame.
but really the simplest option would be to find an MTD 860 or 960 to fix up.