I have never messed with the tensioner on this particular engine, only it's gas cousins, so maybe I can think outside the box, hopefully to give you a fresh perspective.

My first curiosity is whether or not you have the correct tensioner. Do you have the original in a scrap pile somewhere so that you can compare? Do you have the right pulley on the tensioner? Have you used a straight edge to verify misalignment? Are you sure that it is the tensioner pulley and not one of the other pulleys that is the culprit? The straight edge could confirm this.

You don't have a bracket in the wrong place spacing a pulley out or something do you?

I'm sorry I can't help further. I would encourage you to use a straight edge and don't take anything for granted.

When I was a kid helping my Dad in his shop and got stumped I would go begging for help from my Dad. One of his common responses was, "whatever you have NOT checked is probably the trouble.

Best of luck and keep us posted.