Someone like Jurnigan can help you out or any of the other suppliers out there. I have also seen them on Ebay. Most people spend around the same money and call Norman Clay and get a kit to convert it to a automotive starter.
You will benefit in going with a kit from Innovative Accessories "Norman Clay" as Gary stated, the aircraft starter is designed to start when cold, fly, land, not hot starts very often as airboats need, when you remove the starter drive and install the block off plate, be sure that a plug is installed into the drive bushing to hold oil pressure from being dumped from gallery.
Call Norman. He is fantastic to deal with and will have an answer to any question you ask! It's also a hell of a lot cheaper, I just had an aircraft starter on a buddies boat rebuilt due to the new one being $800!