If its not recocking, its not the battery. It would be that theres not enough air for blowback, which probably is the o-ring on the striker. Could just need a good cleaning too.
Just about any reg will work. The older CP that only goes to 500 should work. A spring kit to tweak things would help.
Here's a link for some o-rings, you can also buy them off ebay. I've bought from these guys without problem: