older gun

Nice find. Those are real relics. I'm not intimately familiar with the Mirage. As I understand they use a valve system very similar to the earliest Tippmanns and Montneel guns. My first step would be to open everything up and clean the living hell out of it. Dish soap cuts through old paint remarkably well and using the hottest water you've got will break down old shells. Re-lube with a good oil (KC Trouble Free was always my favorite). Put it back together and see what you get. If you don't have any leaks, you shouldn't need any O-rings. However, if they are chewed up, they might be increasing friction drag while still maintaining a seal. Also, I don't know what material/quality of spring they used, but at 20 years old, the springs may be beyond their life. Interestingly enough, Mokal is still making guns and they actually sell rebuild kits for the Mirage for $30. If cleaning and relubing don't work, it might be worth it to pick up a whole rebuild kit and start your gun fresh. Keep us posted!

Not sure if oringmonkey.com would have it but you could check there, they sell some great rebuild kits.