There's a thing called a Helicoil that is used to repair stripped threads in aluminum, but IDK if they make one that large and installing one will probably run the risk of getting metal chips in the engine, which is not a good thing. A Helicoil looks like a stainless steel spring that is screwed into the stripped hole with a special tool, then the excess is clipped off. They work great. In the case of the chain tensioner it would be imperative that the Helicoil be installed dead-nuts straight (something you've already be unable to achieve) or it will leak like crazy.
