The 1901 monitor may use a cable unlike the one you need. However,
when I had one, the RGB cable plugged into the 1902 round 8 pin port.
You may want to try the following sources of Commodore paraphernalia:
Centsible Software: IBM * C64 * Amiga: for over 18 Years!
:4000 Software titles in stock!
MC/Visa Accepted: Orders: 1-800-640-6211 / 616-471-1089
Fax: 1-616-473-3456 Tech Support: 1-616-471-1083
Cinncinati Commodore Club for sale WEB page
I have never heard any complaints concerning Centsible SW but neither
have I dealt with them. I purchased stuff from the Cinncinati Club.
I found their product description vague. Once they clarified the
items' specifics, the club's merchandise and service was excellent.
You may want to search for a site operated by Ray Carlsen
(Carlson?). It may contain instructions for cable fabrication.