Here's a few checks you can do to figure out if the starter is working.
I use a spare car battery I have. I hook jumper cables to this battery. The other end I hook the ground cable to a foot peg mount nut, a good place to ground to. In the positive cable a put a long 3/8 bolt a screwdriver or something along those lines.
Now with the bike in neutral on the side stand look up under the bike. Find where the cable hooks to the starter. Pull the rubber cover back, touch the bolt/screwdriver to the stud on the starter. If the starter spins the engine over well then the problems not the starter.
If it doesn't spin the engine then pull the starter and rebuild or replace. On the XS2 and TX650 from what I understand the starters were not as strong as the later. In 74 with the TX650A they upgraded the starter and started using a stronger battery.
If you replace it I would recommend going with a 74 or later starter.
If it cranks the engine ok, move up to where the cable hooks to the starter relay. Pull back the rubber cover, touch the bolt to the stud the cable bolts to, this should spin the starter. If not clean the cable connections at the starter and at the relay.
If it cranks then, next I use a large pair of pliers. Touch one side of the pliers to the stud on the relay the cable to the starter hooks, touch the other side of the pliers to where the cable from the battery hooks. This should spin the starter.
If all these checks spin the starter you need a different starter relay.
Cleaning the cable connections all along the power path is a good idea.
Starter problems can be along the ground path as well. The starter grounds to the engine, the engine grounds through the mounts to the frame, from the frame to the battery.
If the case where the starter mounts is dirty it won't ground well. Where the engine grounds to the mounts, any paint or powder coat stops grounding. Here I would suggest removing the rear mount. Just two flat triangles of steel. Where the two triangles touch the case, clean bare metal. Where they touch the frame clean bare metal.
Up where the negative battery cable hooks to the frame, again clean bare metal to bare clean metal contact is needed.
Leo