what's wrong with an old un?
It really is a case of swings and roundabouts, what you'd spend on a new cocker you may well end up doing conversions to your old one. Then again you could buy a new one and spend a load of cash again sorting that out to your own preference. If it were my decision I'd get your cocker suped up and use the cash I was thinking about spending getting it running really nice.
I'm still running an old, (got it in 95) cocker and I've not found a good enough reason to change it yet, (other than those Aurora Eclipse cockers, they're cool!). One of the good things is I can change bits here and there to inprove it's performance.
Back in the days when I ran CO2 Jack at Planet did a lot of work opening up the internals to improve gas flow so I'd recommend that sort of work as the bigger chambers apparently improve performance, (feel free to correct me if I'm wrong here folks I'm no air tech)
Other than that you could think about a blade frame, a new pneumatic set, front reg and change your bolt to a venturi if you haven't got one already.
The good thing here is you're already working with a good platform, (IMHO), add the bits as time and cash allow. But remember the important bit that I always forget when faced with a new gadget.
Once it's working clean it, shoot it and don't mess with it.
HTH