There are various vintage audio site which may have forums or contact lists for parts and repairs. I know there's a few specialist Marantz restorers on the internet. I'd Google the model name and "vintage Marantz" and see what comes up.
What a handsome amp you have --- I'm fond of the 1050 integrated amp I have which must date from the late 1970's, found in the garbage a couple of months ago in decent cosmetic condition and still works absolutely perfectly. Find the Marantz sound a bit bright, personally.