If the starter motor doesn't engage when it is activated then the solenoid is broken or the windings are burnt.
In most cases you will have to replace it with a new or a professionally overhauled starter motor. This is the best choice instead of a used one that you do not know how long it will last.
In the starter motors that are equipped with a starter bush (mounted in the gearbox) it could happen that there was a different starter motor mounted. Check the outer diameter of the old starter motor axis and order the right starter bushing as extra (see tab options).
The 12 Volt starter motors for the manual gearbox are factory new, the 6 Volt model and those for the automatic gearbox are professionally overhauled but meet the same requirements as factory new.
The most common starter motors are also available in a B-grade (budget friendly but lower in quality), if available these are listed under the alternative tab.