Above the sprocket on the motor, there should a black rubber grommet/disc about 20mm in diameter. Pop it out with a small flat blade screw driver. Once its out you will see a 10mm bolt head. Put a socket on it and see if you can spin it, this is on the other end of the shift drum. Sounds like you bought someone else's problems. These engines are pretty simple once you know them. Jump on youtube and start learning. Try searching for Alex Mann, he has done alot of vids building and rebuilding and building up his honda z50r, internally just about all auto clutch pittys are the same as a honda z50,xr50,crf50, etc. Once you have opened em up a few times it all makes sense.
I reckon you have a few problems. First you said it was hard to put into gear, could just be clutch adjustment. But then you said it locked the rear wheel makes me think its inbetween gears, possibly the shift star screw/bolt has backed out or the shift arm off the shift shaft has moved/jumped out of its groove between the shift drum and shift star. Take off the Kickstart, thrn the clutch side cover and jave a look. You will see where the gear lever shaft comes through the cases from the other side, there will be an arm coming off it going to the shift star/drum at the top of the motor above the kickstart. There is another splined arm that goes to the clutch to engage it on gear changes. Any of these issues the minimum you will need to do is remove the clutch, gear lever and the large gear next to/driven off the clutch. You will need a manual impact driver,clutch nut tool and circlip pliers.
Only way to find out is to rip off that cover and take a peep. Watch some youtube vids, like i said once you get your head around these motors you can build them in your sleep. If you aren't sure about something take a pic, someone will chime in with the answer or solution