Man, I don't think you can replace just the HEX BOLT looking piece. From what i rememered, it was a solid piece onto the whole triangular looking metal piece which would involve taking a lot of things apart.
I had also stripped the bolt because fools at the dealership gave me the GT Belt instead of the GT-S which is 1.25" or so shorter. I kept pulling the bolt to get more clearance then it had to give.
My solution now is using a huge clamp and wrench to pull down the tensionator.
Notify me how easy it is to change that Hex like bolt, maybe i will attempt to do this next time I have to do changes in that area...