Sorry to hear that, its a right pain. As for the crystal, all the info you need is here: http://www.thewatchsite.com/21-japanese-watch-discussion-forum/35635-7a28-7039-crystal-replacement.html As for crystal gasket (nylon?) they are hens teeth on older Seikos, the part number is 86566600 and was used on 7A28-7030/ 703A/ 703B as well, try to use the one you have. If you do use a generic gasket you will probably have to use a slightly smaller crystal, as the generic gaskets are generally thicker. I have used a generic gasket on a 7002 diver with the slightly smaller 7S26 crystal and it fits well but its not been pressure tested.
Nice work so far, looking forward to seeing the finished item.