Old systema parts used to be fantastic new systema parts are not known to be so great these days.
Prometheus makes some fantastic quality parts for all your internal needs.
As far as hopup units go I think ICS has their own style hopup unit so I am not sure if a standard hopup unit would be compatible. However you should be able to user whatever rubbers freely. I personally use a guarder clear but my M4 only fires as 380fps 0.2g. Perhaps a Prometheus rubber.
"Its only a little bit on fire"