never heard of a timing chain streching. Is it loose enough to jump teeth on the cam? if you have removed the head ,you may not have got the chain back in it's proper position in the guides or the chain was not positioned on the lower crank gear correctly. Normally the chain tensioner can account for any sort of stretch. Check the tensioner to insure it is extending correctly. Facing the motor from the rear, on the left of the carb ,there is a unit bolted down to the lower case with two bolts and a cap between them. This is the can chain tensioner. remove the cap. look down inside, you will see a slotted screw head. turn the screw clockwise until it stops. This pulls a spring loaded plunger back off the timing chain,locking it up, relaxing tension. Turning the screw counter clockwise unlocks the plunger allowing the plunger to extend, putting pressure back on the chain. You can remove the tensioner by unbolting the two bolts and pulling it up out of the case. while it is out inspect it and turn the screw to insure it retracts and extends freely. lock the plunger up before removing it and don't forget to unlock it after re- installing. I have made this mistake before and got my cam timing all screwed up!
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