Hi, 
  I'm brand new here but I have a direct injection 1650. If I understand correctly there are two starters used on the 50 and 55 series tractors? one of them turns at a much higher rpm than the other? I believe my 1650 has the low rpm starter and my 1955 has the high rpm one. When the motor on my 1955 went south I used the 55 starter on my 1650 for a while. The higher rpm starter made the 1650 start much better.  Would it be possible for you to locate the high rpm starter?