Graham is right I think about the box unless it's been altered, here's a helpful link which shows the clutch setup and deals with potential wear points and improvements.
http://www.gomog.com/allmorgan/plus4clutch.htmlDon't know about the availability of parts and would appreciate some feedback if you go ahead
The GOMOG article is written by Mike Miles (A Professional Engineer) who lives in Portland, Oregon. I know Mike and I have re-read the article in GOMOG. Mike is right on. Mike has a lot of experience regarding the drive train of the TR powered +4's. Be sure to thank Lorne Goldman for including Mikes article in "GOMOG".