The adapters are readily available, and I bought one for my 635. Make sure that you are buying a complete kit, which consists of the white rollers to hold the canister, the plate, and the take-up reel. The 35mm plate also includes the sprockets for the film.
For 35mm, I'd rate the Yashica lens as being good enough. I can't imagine anybody buying a 635 just to use it as a 35mm camera. I bought my 635 to use HIE 35mm without springing for an XPan. A bit of tape to lock to take-up spool in place, and there's some film wasted because the 120 friction counter doesn't engage on the 35mm film.