FarmallBob, the original starters for all Gravely T-head engines were Delco Remy. I once heard you can take the guts out of a regular DR starter (like a Farmall) and do a transplant into your Gravely starter. I believe a few starters had the same body, but you would reuse the front flange mount and gear drive that the Gravely needed.
But I haven't done that transplant business to know if it works or not. All of the Gravely starters I have played with were not hopelessly gone, and were able to be rebuilt by a starter shop.
www.gravelyparts.com is Richard's Gravely. He used to have rebuilt/new starters available.
Here's a link I had in my bookmarks for a not so cheap version: http://www.amazon.com/STARTER-GRAVELY-TRACTOR-L-MODEL-1108311/dp/B0079PI7TA
Is the Delco tag still on your starter? Would NAPA be able to match you a starter with that information?
Ben W. in VA