starter

If you can hear the starter running but it does not engage check polarity.

To be honest, I have „tested“ that myself one day, and surprise, if polarity is flipped over the starter turns nicely but does not engage due to turning in the wrong direction.

in case that helps, don‘t tell anyone...

Peter, that is interesting, it is not frame grounded?

Check polarity at the battery (not the starter). I once had it taken out and fitted back with the cables flipped around. The effect was that the starter turned (but the wrong way) and did not engage. 
We suspected virtually everything as possible cause and even intended to exchange the starter to a new one. However, the solution was to connect the battery correctly and all was fine. Took us a while to accidentally find the true fault.