The problem is either with your solenoid or a poor ground.  Both are
fixable by the average Joe (or Jane).

You can run a ground from your battery directly to the starter using
wire from a set of old booster cables.  The solenoid contacts get dirty
& corroded, but you can fix them using a screw driver, a soldering iron,
and some sand paper, I've done it a few times now.  Search the archive
under starter or starting motor and my name.  I have described how to
make this fix before.

Cheers & good luck.

Bob Norman
'81 Westy
Gander, Newfoundland
Canada