Just looking through the article on the site by Mike Viccaro.
Good article, good info.

I just wanted to add two comments. Mike suggests testing the starter after repair, a good idea but please do just a momentary test. Starters are not made to be run without a load, if left to spin at high speed the can damage themselves and you. Due to their high torque unladden the armature can start coming apart from centrifical force. So for a test please just a short momentary test to see if it spins up and then disconnect power.

A second suggestion for reassembly of the brushes, they can be damaged by screwdrivers. a better assembly technique would be to tie the brushes back against their springs with light thread. Dental floss works well for this, once the brush housing is over the comutator, cut the thread and pull the string out.

I checked Ebay today and there were over sixty hits for Kubota starters. The price ranged from $80 to $170 for new units. This could be another option if you did not want to pay the Westerbeke price, I have heard that is near $500.