The 3L-310 V-belt is the correct replacement belt for the Raynor R120 opener. As far as remotes you'll need to let us know what make/model of radio receiver is attached to the opener to help you. The older openers like yours had the radio equipment added by the installation company when the opener was installed so we don't know what is on your particular opener.
If you have an existing remote that works there may be a make/model printed on the back of it (if it hasn't worn off). If that's not readable then you'd need to get the info off the back of the radio receiver which is most likely attached to the back of the operator.
You can also email us with pictures or other questions at: [email protected]