You do know that they make carb kits that have replacement throttle shafts, new shaft seals, new throttle plate and screws, etc don"t you? They are ~$37 plus shpg on this site. Click on Tractor parts in the upper left hand corner of this page and cleck on Massey Ferguson and then Fuel System. That"s a bit cheaper than ~$250 for a copy cat replacemnt carb that"s probably not as good as what you have.
Get a can of carb cleaner ands disassemble the carb and soak the carb in the cleaner for a couple of days. Blow out the passages and make sure they are clear of crud and rebuild it yourself. It ain"t the Spaces Suttle! It"s about the most simple carb ever designed.