Older Stanley brand openers can be pretty common and they were like the first opener that you could buy at a hardware store and install yourself. And believe me they were built to last. These brand of openers is made of entirely plastic so if there is ever a problem with the garage door coming off track or twisting about, it can easily break its plastic trolley. Sometimes where the chain attaches to the trolley it can break at that point but also where the hockey on that attaches the trolley to the garage door can break off also. Even though most of these openers are very old they still make aftermarket replacement parts for them but it is limited they don’t make all the parts for them anymore just the ones that tend to break more often. Replacing this part can be somewhat easy if you are mechanically inclined you just have to disconnect the chain and the hockey arm from the broken trolley and remember where all the parts go which are not many and you should be ok. Once you have been disconnected all of the parts attached to the trolley then you will need to slide the old trolley off of the opener rail, which is almost always tubular.
Rail is in three section
Most of the time the rail is in three sections and all you have to do is loosen the garage door opener from the header beam or the wall where it is attached at the top of your garage door once you do that you can just by hand separate the sections of the rail and remove the old trolley and slide on the new one on then you will need to connect all of the parts just the way the old one was like the Hockey arm and the chain assembly. Remember to put the trolley in the same position on the rail as the old broken trolley in the limits up and down should be pretty close to where they were before you started to work on the opener. Not then the limits on the up and down will get mixed up if you do not put the trolley exactly where it was when you disconnected the chain and hockey arm they would have to be adjusted so you would have to know Approximate position. Far as finding the replacement part for these brand of opener you will have to search online for a Stanley opener replacement trolley and you should be able to find a company that carries one I know that there are several out there and have had to order them before. You have to hire a garage door technician to do this for you then let them know what they are dealing with so they can order the part because it could take up to a week or so to receive the part.