Clean all cable/wire terminal connections pertaining to the electric starter, including the actual battery terminals themselves. Also clean the components that the cables/wires attach to. Tighten all tightly with a wrench or pliers.
Check the battery with a "Hydrometer" to make sure you have no weak or dead cells. Make sure the battery is good and fully charged.
To test the electric starter.... have a good known fully charged battery and use known good jumper cables directly to the electric starter. If that cranks the engine over okay, then obvioulsly the starter is okay.
Yes, electric starters can be rebuild when needed.