I've still got two GTs. I don't find parts particularly hard to find. They're all over e bay. If you're changing out the black tub chassis for the blue flat one you'll need to get a new motor mount. There's a difference between the pull start and non pull start motor mount so you'll have to be aware of that. the pull start mount is about 1/16" taller to clear the pull start. It's been a long time since I've switched mine but I think the rest is pretty much just switching everything to the new chassis. There's different blue chassis lengths. The factory chassis is a little longer than the rtr chassis. They both mount everything the same, though. I like these old trucks. Edit Just remembered you'll need to get a set of nose tube mounts, too.
https://www.rcdocuments.com/document...0GT_Manual.pdf