What snapped/broke is the rocker arm stud. Looks like you may not have loosened the jam nut, you should have held the pivot (larger hex) while loosening the jam nut (smaller hex). You should be able to replace the stud (maybe have an experienced small engine mechanic do it). I suggest contacting GPS/MMP about buying a stud since the Lifan/MM head was supposedly a proprietary MM item (only the MM-B212 has that head according to information I've seen). See if you can remove the pivot and jam nut from the broken stud, if not you will need new ones. I'm not an expert on small engines so seek information on how to remove the broken stud and replacing it from experienced engine builders on here.
@ole4