Hello, Edward -
I have only been able to find Service Manuals for the Marantz SD-160 dual deck cassette player for sale.
I was unable to find the operating manual for the vintage cassette player / recorder on the Marantz website.
However, if you have not yet already done so, I suggest you contact Marantz Support, either via phone or email, and ask if there is a copy of the operating manual for the SD-160 in the company archives and if they could provide you (sell you) a photocopy or scanned PDF version of the manual. This is the link to the Marantz Support page:
If that option doesn't work out, Option 2 is to find one or more online sellers (eBay, Craigslist, Amazon, etc.) of the model of cassette player / recorder and see if they mention the operating / owner's manual in the product sales information. If they don't mention a manual, I suggest contacting the seller/s and asking if they have the manual.
If you find an online seller with the manual, ask if he / she would either photocopy the manual for scan it as a PDF document and sell it to you.