As the man says above, just shortening the existing spring slightly may well have the desired effect.

I have a WonkyDonky kit in one of my 99s and it's lovely. You could approach him and see if he has a spring. I could well be tempted to ask him for a .22 spring, sacrificing power for the even milder cycle that you are after, although some would say that it may drop the power too low. Worth a try in my opinion and you could add a little preload if the output was too low.

If you have the spring out of the gun at present, you could check its measurements against some of those stated on the Mainspring Data Thread in the Useful Tips section.

If choosing a new spring, don't forget you'll need to make sure that the guide (and top hat if desired) fit properly.