Is the bendix engaging when you turn the key? Does it engage when you jump across the posts on the starter relay? Any click-click sound? If no sound of the bendix engaging it could be the bendix is shot, the starter relay is shot (bypass it to find out) or no juice getting to it, to engage the starter teeth to the ring gear. If it is engaging the bendix then the contacts in the bendix that feed juice to the windings to energize the stator and spin the starter motor could be corroded/pitted/worn out, the brushes could be used up/chewed up and / or there's a short due to brush dust accumulated.
Starters are DC motors and pretty simple to tear into. I've polished the stator contacts, washed the brush dust out of a few and cleaned/sanded/filed the bendix contacts on more than one to it going. They can be bench tested by clamping it into a bench vise, and hooking jumper cables to it. Positive to the + on the battery and the stud the heavy red wire bolts to. Negative to the - on the battery and to a mounting ear on the case. A bump switch, used to bump a motor over when setting valve lash, between the wire posts on the starter, or even a screwdriver...