Alternative to a 50S...
You can find a rifle with similar power, accuracy, weight and size, but you won't find one that has all that plus the absence of ANY plastic or synthetic material visible without disassembly, and even then, it's either gonna' be a seal, or a bearing surface for the cocking linkage. A you certainly wont find another springer that combines all of those desirable features, but then goes one giant step further and gives you absolute control of the rifle by fitting it with a Rekord trigger. If you're not sure why that's important, just Google " Rekord trigger ". It's always been the best sporting trigger of any piston powered rifle. I've seen some that come very close, but the Rekord is the one that I measure all piston gun triggers against. Match gun excluded, of course.
I guess my point is, there really isn't a valid alternative ( assuming that means comparable ) in its class.