Cam chain tension is released by turning the screw visible through the top of the tensioner, under a cap. Turning the screw raises the spring loaded plunger, taking pressure off the chain.
Turning the screw back opposite a couple of turns releases the plunger allowing spring pressure on the chain.
Normally you can hear or feel the plunger pop free when released putting pressure on the chain.
I can't remember which does what, turning the screw clockwise or counter clock to lock and unlock. Best way to understand is unbolt the tensioner from the case , turn the screw and observe what happens. if the tensioner is not manually released it's gonna jump timing.
You may be missing the 2 centering dowels in the head. Those are little tubes that fit diagonally in two of the four long head stud holes They align/center the head on the cylinder so it won't shift when torqueing down. Facing the head with the intake side up, upper left corner and lower right corner.
The dowel tube in the upper left corner is very important .It serves double duty of not only keeping the head centered but this stud hole also serves as the main oil feed path to lubricate the top end.
Oiil is pumped under pressure up through this stud hole to the rocker assembly then drains back down through the cam chain opening into the crank case. The dowel keeps the oil pathway aligned. You can see looking at the mating surfaces of the head and cylinder the two holes that are machined slightly larger to accept these dowels.
I don't think you actualy stated that the head has been milled. If the surface was not milled correctly, cut at a slight angle, this too can case the problen you describe.
If it were mine I wouldn't chance that piston. it doesn't take very much pressre to beat the connecting rod pin holes out of shape on an aluminum piston. Smacking a valve hard enough to bend it is enough to misshape a piston I would do anything I could to keep from going back in there again!
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