Clean the outside off really good first...use some spray carb cleaner or something like that....then take the head off and check the top of the piston and see if is are scorched or looks blued...if it is the motor is probably shot...also check the piston sleeve and see if it is majorly discolored and/or scratched up...then clean the seating surfaces on the block and head real good and use some high temp RTV (orange)...not TOO much..just a very tiny ribbon on the seating surface of the block..you don't want a bunch oozing out inside the motor..when you put the head back on don't crank the head bolts down TOO tight just make sure they are good and snug...make sure you tighten them down in a star pattern..(criss cross)...not in a circle...tighten each one a little at a time until they all seem equally snug...while you have the motor out fixing your pull start take the backplate off the motor and clean the seating surfaces for that up real good and apply some RTV to that before putting it back together...

Good luck and keep us posted.[8D]