I've had 4 platens recovered on my machines, 2 from JJ Short, 2 from Ames (no longer in business). All 4 were wonderfully done, and make a huge difference in the machine's performance. The cost is not much, like Amelia reports, it's between $60-$100 depending on the size of the platen. That's actually pretty close to what Ames was charging for many years.
Regarding platen diameters, the only source I know of for "factory fresh" diameter measurements for most platens is the 1962 edition of "H.F.W. Schramm, Liste der Herstellungsdaten deutscher und ausländischer Schreibmaschinen, 11th edition, Hans Burghagen Verlag, Hamburg". Not an easy thing to find, but I'm slowly adding those measurements to the Typewriter Database serial number lists over time. If you are thinking of having a platen recovered that hasn't yet been updated in the Database, go ahead and ask me (provide manufacturer and model), and I'll see if it's in Schramm 62.