Adding a data point since I know people will find this thread searching.
I bought TWO of the Ebay tensioners I linked above. I figured that for $67 each shipped, It was good to get while the getting was good. Especially considering when my original one let go a few years ago, the only game in town was the billet White Marine unit for $500.
So, I have two spares.
Notes: This is our tensioner. Casting looks great.
The pulley is wrong. It's 90mm. So, you'll need to source a proper pulley or get a longer belt, but the tensioner appears to be legit.