Hi. I’ve also got one and really enjoy it.

Here is a link to the Miyata Catalogs of the 70’s, 80’s and early 90’s: https://www.ragandbone.ca/Miyata/miyata_selector.html

Here is a very long Miyata thread from this forum: Just how big is the Miyata Cult?

You won’t find a repair manual but most vintage bikes of similar eras more or less work the same, and there is information aplenty between this website and the rest of the internet on how to service bicycles. Here is a good place to start: https://www.sheldonbrown.com/

Other than that search Bike Forums for various repairs you need to do and you’ll find a thread on it, or you can start your own.