What makes you think you need rings. When you say you dropped a cylinder, do you mean that there is no compression in that cylinder. If so could be burned valves. This engine is not for the average mechanic to go into and rebuild, at least that is MHO. Could also be a bad coil pak, I had to replace at least two on my 960, before I went ahead and replace them all. Wiring to the coil paks is less than robusk, this cause numerous instances when the engine would miss.

To go to install a turbo in that car the wiring harness will be a major issue.
I have a 91 940SE, which is essentially the same car with a turbo and AW71 tranny. My 960 is now a parts car for the 940SE.

I inquired about putting a 302 in the 92 960 I have after the timing belt broke and was told at that time that a wiring harness could be made but it wasn't cheap, around $1,000. Of course that was several years ago.