I have rebuilt the carb, but it was rusted shut and I have gone totally through this motor, and I don't want to cut corners on this. I took a carb off of my other A and the tractor ran great!!!!!!!!!!
If you A is Gas only then it is the Marvel Schebler TSX-157 (I just re-built one), IH Part # 10.2657 and if it is a Zenith (optional) it would be 52499DB.
I would first get hold of say Ken Updike at Carter & Grunewald, or you local CaseIH Dealer. They should be able to find the carb for you.
You can also contact Jamie at JP Tractor Salvage, as they may have used Carbs for your A.
An early A would have a Zenith carb, I'm not sure when the Marvel carbs became availlable, but Jim likely has that information. super A's may also have had a Carter carb, I know some C's did.