Spring guide should be replaced at a minimum. You will need a V3 Gearbox's spring guide, one with Bearings, and metal preferred.
That would be the "At a minimum" replacements...
Personally, I'd just assume you take the air nozzle (AUG), Cylinder head (G&G Proprietary, ASGI has replacements with new piston and piston head), piston and piston head and replace them too while you are in there. Depending on how crappy your CA motor is, maybe one of those too (Short motor). Needless to say, neither of my two CA M14's would pull a Guarder SP120 reliably on the stock motor and a large 8.4v 3500 mAh NiMH. It's easy to spend other people's money though. Is it a requirement to replace those? No, but I would just to ensure maximum performance...