I would try new chain gudes first because if the solder comes off look out.
Is the tensioner all the way out and the chain still loose?
Also try a thicker base gasket to reduce the chain slack.
I have a chain tensioner brand new that I could'nt use because it was too long for my stroker motor and the chain was already on the tight side until I shaved .015 thous off it to make the cam sit even in the head.
All the tensioners for my gy6 motors are 43mm and this one I got from mbne is 50mm.
This is measuring it from the base to the tip of the tensioner
Let me know if the picture link doesn't work.