Timing a 125cc pit bike up

GEARBOX SELECTOR IS WORN/DAMAGED

its probly worn out--thers an ajustment-u can try that----or get a prorepair book on that model----dont keep kikn it--u cud bend a valve from the piston being out of time

remove the cover that houses the kick starter and look for a failed return spring or some other reason that keeps the kick start dog engaged.

dirt in carb

clean the carb !

ignition or valve?

After hunting around the web for a few, it sounds like the spring is from the timing chain tensioner. It looks like there is a button on the end of the cylinder, that end goes in first, then the spring. See if the spring is the same on both ends, the picture looks like it's smaller at one end and that end goes into the tensioner, not the bolt. I'll include a screen shot of the picture.
Good luck!

CLEAN AND OR PLACE THE SPARK PLUG.YOU MIGHT HAVE A WEAK SPARK

Try unpluging the cutouts.they are usally on 2-3 places on these bikes ,brakes,also check the fuse that is on the red wire comming off the battery it will be blown or end of fuse holder will be pushed in and not touching the fuse.