Cool machine!
You probably want to keep checking eBay for parts, and maybe build a custom search to notify you. I did that when I was looking for the free play mechanism for an old Mills gooseneck machine and finally found it.
Dave, the guy that runs this site, has a cool set of DVDs that document the tear-down of an old Mills Castle Front, and they are really helpful. They would have saved me a whole lot of time if I'd known about them way back when.
I'm working on a site now that documents my own tear-down and restoration, but right now only the disassembly is online. You can check out my own modest efforts at
http://www.slotrestoration.com. There are a few parts resources listed on the site as well.
Congratulation on the machine... it will be a fun project project for you, made all the more special since you have a family connection to it.