I replaced the piston rings in my daughter's 92 SL2. I took the engine out though, along with the tranny since the tranny had the hole in the differential from a lost diff. pin. With the new rings it went from being an oil guzzler to using less than a quart in 4000 miles - which is when I do the oil change anyway. I didnt need to replace the pistons, nor even remove the ridge at the rim of the cylinder. I could still see the honing marks all the way up the cylinder wall. This engine had 128,000 miles on it at the time. I also replaced the engine bearings (though I didnt need to), and of course the oil seals on the crankshaft. I also replaced the tensioner for the timing chain. Nothing else needed changed out; not the timing chain, not the guides.
I got a tranny from a junkyard, and put that in without any fixing either. It works fine.