Looks like a stock piston. It has the small flat circle on the top which certainly the Mahle pistons I used in the past did not. May or may not be accurate Really would need to see it from the other side to tell!
HD Chain and Tensioner with higher spring presure. Part of the issue with stock chain is that it is quite weak, and is likely to stretch, especially with stronger springs.
I have done this many times over the 10 1/2 years I have owned my BA, and have never had any issues. Worst problem is that the serpentine belt gets a bit dry, but a couple of squirts of silicone oil or tyre shine fixes that. Maybe the general use sh*te electrical connectors!
Good news Phil, good thing you caught it early and stopped in time. Yes the Atomic Cr-Moly flexplate I fitted to my car was around $600 incl. ARP bolts for supply only, but it is a nice piece of kit, and more like a flywheel than a flexplate.