if you buy ruger sr-556 you will get your piston wish.
as far as retrofits go well designed system shouldn't care if your receiver/barrel DI or not. just couple of those out there - adams and superlative. both seems to get decent reviews.
gas piston systems have one requirement - tightly fitted/enlarged tail section of the carrier to prevent tilt and digging/rubbing of the carrier into the buffer tube. no special voodoo here.