I have 3 Gravely 800 riders and have not had to replace PTO clutch yet. My understanding is that changing the cone is dangerous and difficult and the recommended fix is to replace the whole assembly, I think it was $250 several years ago.
Gravely parts lookup
Select Gravely in the pull down near the upper left (it comes up Ariens) to help with part numbers if you don't have the parts manual.
You can get most manuals (pdf downloads) from
OldGravelys.Net: Gravely Manuals
One of the many Gravely parts sources at
Gravelyparts.com
I see the PTO stuff on page 8, looks like prices have gone up ($399 for whole kit)...
Ebay is a crap shoot, you don't know if you're getting a PTO clutch with life left or a worn out one... if you use eBay, just be sure to check seller feedback and ask questions before bidding (of course you already know this)
Good luck.