When the starter just clicks, usually the brushes have worn down, especially in higher mileage first gens, definitely with the original starter.
You can be extra sure by tapping the starter the next time it won't start. Use a block of wood or a rubber hammer, something that won't damage the starter housing. If it starts up after you whack it, you can be sure it's the brushes.
You can get the brush assembly inexpensively if you know enough to partly disassemble the starter, or know someone who does. It beats buying a new one for $100+ or getting a cheap China-made one.
If it's not the starter itself, it could be either the starter-mounted solenoid, or the fender-mounted solenoid near the battery. Both are also inexpensive and fairly simple to replace.
