Finding the Part Number for Vintage Equipment

I bought a TEAC A-700 cassette deck that was not functioning. After replacing some belts (the general set associated with the model) and doing some general cleaning, I determined that the problem was the idler tire being worn out. I have the model manual, which has helped me get this far, but the manual also doesn't have a part number to do a search. Is there some way of sourcing the part number?

Second, I did a rough measurement using calipers and tried to search for a generic replacement. I get that the ID needs to be exact, but how much give can the thickness and OD have?

Help on either of these fronts would be appreciated, thanks.