M50 swap, M42 Air Conditioning

[indiejack]

First off, this is the first time (and will be the last time) I've ever let someone else swap an engine in any car. I got a deal on the swap and labor for around $1200 and I figured "Hey, less headache as I've never really dealt with BMW swaps" but this guy drug his heals for weeks on it and in the meantime I was driving his spare 5-series wagon (that was constantly overheating and was one of my first tips that this guy probably was going to do some shotty work).

At any rate, engine is in and running strong, I need to run down the CEL that's related to the O2 sensor either being bad or not properly hooked up. He put my old working AC compressor onto the M50 and ran the lines but told me that I needed to source a different tensioner in order to get it to work. My question is: Is this correct, if so what do I need to get because just getting any answers out of this guy is like pulling teeth and it's 103 degrees here today!?!

Thanks in advance for any answers, I searched but didn't come up with my specific question, they were more related to running the lines.