Curious as to what you are experiencing to think your starter clutch is toast(ed)?
I have not had to do one yet (knock on wood), but in my opinion, before I split the cases I would rebuild my starter first, replace the CDI with a new one, check the stator, switches, coil etc for functionality/resistance and maybe new spark plugs. Might save yourself a lot of bench work. I had trouble in one and found the stator was bad. I replaced CDI's too and my bikes have been faithfully turn key since.
Have you done a full tune up and confirmed you harness is communicating properly? With these bikes being almost 30 years old, I would recommend checking everything (if you haven't already if course!
Might you have more details regarding what you are experiencing?
For starter clutch parts, If I am correct, my research shows that you will likely need to replace the internals only, #7, 8, and 12 and it looks like they are quite available:
I have never had trouble finding parts on the ol' webernet and have picked up 450's for a song.
Hope this helps!