To me it sounds like the motor itself is OK, it whirs up to full speed quite nicely at one point. Since you always at least get a click when you push the button, the button and harness circuitry for it are OK.
Cicking and a lack of motor action points to one of two things: the motor or the solenoid relay. You could test the solenoid by just jamming a beefy screwdriver between the nuts on the two solenoid terminal studs. The nuts because going between the threaded posts tends to bur and damage the threads; a burned pit in a nut doesn't cause much trouble. If the motor spins (or you get that clunking) reliably every time you jump the stude, but just a click and no action with the starter button - that says the solenoid is flaky. If the motor doesn't work with the studs jumped, then the motor has issues.
The solenoid is serviceable. You disassemble it, clean the contact points (the ends of those studs) and the moving contact, clean any gunk out of the casing, and reassemble.
The motor is also serviceable. Usually it just needs a set of brushes and a good blasting through the brush holes to get carbon dust cleaned out. Only if really worn out brushes were ignored and the motor used a lot in that condidtion is the commutator damaged. If that's happened I would source a good used motor. Getting it apart, dressing the commutator, and seating brushes to the new diameter takes a lot of time and is it really worth it when motors are fairly easily available?
But: this sounds like starter clutch to me. Rather a common problem. You have to get in there and have a look. The clutch is between the alternator rotor and the starter driven gear. The "outer" part on the rotor usually is the problem. There is a heavy housing screwed to the rotor. These screws can loosen (especially after poor tightening and no staking when someone replaces one) which usually causes the housing to crack. In the housing are three rollers, three springs, and three spring caps. These can come out somehow, especially the springs and caps. When the rotor is spinning (motor running) the rollers are forced out by centrifugal force and don't touch the other part - the driven gear. This is pretty foolproof except for the bushing on the crankshaft. It's oiled but can wear out and get sloppy.
The mechanism works by having the rollers, pressed against the gear extension, jam against it when the gear turns. This locks the two together in one direction but slips the other (the crankshaft can't drive the starter motor forward but it will drive the motor in reverse should a backfire while starting spin back the engine a bit).
The starter clutch parts are hard to find NOS but there are of course used parts available.
