Personally, I consider the starter motor terminal a one time maintenance inspection item on any bike I pick up, so...
There's supposed to be a nut under and on top of the terminal. If the under nut is still in fair shape, it's a simple matter of putting a spanner on it while you loosen the top nut. Clean everything up real good, maybe replace the terminal and nut's-n-washers... install a new boot and you're good.
Top pic: before disassembly. Notch the upper nut to drive it off with a punch
Middle pic: sad state, eh?
Bottom pics: All better
View attachment 309533
View attachment 309536
View attachment 309535
View attachment 309537