The clutch is internal to the transmission.
The procedure to replace the clutch is in the manual at
http://oldgravelys.net/pdf/Pro_G_Serv_Man.pdf starting at page 14.
Better pictures are in the manual at
http://oldgravelys.net/pdf/8000_Series_Serv_Man_0385.pdf
Here is the Readers Digest version as I remember it.
Drain trans oil
Jack up right side only and rest tractor on jack stand. Leave left wheel on the ground.
Disconnect mower deck PTO driveshaft. Leave mower connected to tractor.
Remove right side of rear hitch
Remove engine, right wheel, right wheel hub, RH axle block, brake drum, right side direction clutch, (not sure if the left (reverse) direction clutch needs to be removed) direction clutch shaft snap ring, detent ball and spring, etc.
Remove side cover making sure the shafts stay in the transmission.
Remove shafts, gears, etc listed in manual.
Remove PTO clutch and shaft assembly.
Send PTO clutch and shaft assembly to Richard's. They will install the clutch on the shaft for a modest fee. It is easy to break parts on that and the spring is dangerous as well. Have Richard's upgrade it by installing the 1000 lb spring.
Installation is the reverse of above. Do NOT use the bolts to draw the cover in place. It should fully seat by hand. If it does not, figure out why.
Do
not use gasket sealant on the case gasket. Use some grease to hold gasket in place.
Test transmission. Install hub, drum.
Install engine.
Install wheel.
Fill trans with 6 quarts of oil.
All done!
Call Richard's for current prices of PTO clutch parts. Talk to Jamie and tell him Richard from Texas told you to call.
If anyone says to remove the transmission and lay it on it's side, don't do it. I have done it both ways and will NEVER lay a transmission on it's side ever again. It doubles the amount of effort and grief involved.