I replaced my OEM tensioner with a custom one I made since I couldn't get the belt to tension correctly. Not sure why but I think it might have had something to do with the lack of AC. I tried different sized belts but what I needed I couldn't get. Anyway, I'll check to see if I still have my OEM tensioner laying around somewhere.
Also, a turbo might be a difficult thing to find... Do you have the exhaust manifold? The ONLY CT26 that will bolt up to the stock ST165 is a single entry exhaust housing off an 87-89 Alltrac. None the the MR2/ST!85 twin entry or Supra single entry turbo exhaust housings will work. And the housings are normally always cracked near the wastegate flapper.
I have spare twin entry parts but that won't really help you. People have made the twin entry turbos work but you would need a spacer, a twin entry manifold (OEM or aftermarket), custom oil feed line, and some exhaust work done. I made the mistake a long time ago of getting a twin entry turbo and priced out the "upgrade" and figured it would cost about $400-$600 not including the turbo.
Give James a call at Performance Techniques and ask him if you might have some leads on turbos. He rebuilt/upgraded mine and I have been very happy. He knows a lot about turbos and might be able to find exactly what you are looking for...
http://turbocharged.com/main.htm