Fixing the leaky pto shaft
Once the shaft assembly is withdrawn from the rear of the tractor, the housing can be taken off and the seal replaced.
I first removed the inner (front) circlip, then drifted the shaft with bearing out toward the front, leaving the seal in the housing. Next the other circlip was removed and the seal punched out toward the front (although it could come out the back just as easily).
The bearing sits on the shaft between a shrunk-on collar, which is the one toward the front of the tractor, and another removable collar (which I did not need to remove). The bearing locates in the housing between the two circlips - you can see the grooves in the housing where they go.
The seal is installed just behind the rear circlip and seals against the rear collar on the shaft. Its inside diameter is 1.625" (1 5/8") and its outside diameter is 2.441" (2 7/16"). The original thickness is 7/8" but this is not critical. It could be replaced by two seals 3/8" thick,   one that is 5/8" thick, or even just one 3/8" unit.

(I have looked into part numbers for Chicago Rawhide seals and the part number is CR16189 for the 5/8" thick one, which has a double lip, or CR16201 for the one that's 3/8" thick. You could also get the CR16204 that's 0.64" thick but it's a special order and quite expensive. Putting in two of the CR16201 would cost far less and get you the double seal, if you wanted)

Note this shaft assembly happens to be from a Ford 9N; they're the same as the Ferguson ones as far as I can tell.