Why systema internals? I guess I feel as if they (aftermarket parts) are overpriced, and not all that great.
Might be best to count your losses depending on how bad your m4 is. However parts are everywhere. I'm sure sogo has plenty, and could get you running up to par, but if it's a complete nightmare, buy a kwa, because you won't see a thread here about what should be upgraded... And if you do, they're crazy.
And yes, I'm a kwa fan