Welcome USMC3533!
I have two links that
may help:
First one is an EBay list for a 478, so maybe that guy has a possible source and/or spare parts.
http://www.ebay.com/itm/GMC-Toro-Flo..._qi=RTM2247626
Second link is also EBay and lists a starter for a 423 V6. It's just a shot in the dark as I'm not familiar with those engines so don't really know if it would work. Hopefully you'll get another response. Maybe that vendor has a source.
http://www.ebay.com/itm/CHEVROLET-TR...UAAOxy7S5SAlZE
Good Luck and post some photos of your boat and engines when you are able.