I guess that the RS is only RS compatible.
All / Most AEGs use thicker grips compared to the real deal in the AK range. The reason I can not definitely say all is due to the fact, that th Real sword AEGs use standard width grips (with thinner motors), and some conversions (like the Yugo M76 DMR) can use the original pistol grips due to their thickness (in the case of the M76 it has to be thinned down to 1mm thickness, and some part of the motor cage has to go too).
Why was the Cyma cage incompatible with the SRC gearbox? If you tell us the reason it did not fit we can give you advice if it is modifiable to fit.
When will people learn to search the internet?
Took me 5 minutes to find an ICS AK motor cage and an SRC G36 motor cage (both are for ver.3 gearboxes) on EVIKE.