good old soap and water for cleaning out mud and clay -
be sure you grease everything well when you put it back together - including the seal and the pivot stud - this will help keep mud and water out in the future. If well maintained, you can submerge these old machines in mud and water with nothing getting in. The brake drum cover must have a good seal though. any tear or rip, and they will get dirt and mud in them, giving you what you have now. Also, they have a drain plug in them - check to be sure it is in and tight.
RIP - Yamahondaman!! You will never be forgotten!
RIP - Sam Brehm!! Gone but NEVER forgotten!
RIP - Sandpuppi101 - You will live on in my mind - I miss you friend!