I have a 1985 yamaha yz 125 wont start

By what you have written,I take it the motor still turns over.Take the spark plug out and see how much compression you have.If there is very little,it may have been the piston seizing.When they cool down,they can sometimes be moved again.This is the most likely cause of your troubles.If you have compression,take the s/plug out,put the bike in first gear and see if you can turnover the motor AND g/box.What you are looking for is a broken part in the g/box that locked up the driveline.Do this in each gear.Sometimes you can take the pipe off at the head and see into the combustion chamber through the exhaust port.Otherwise the head needs to come off.Hope this helps you