Ditto that aftermarket pipes will run lean with stock jets. You may want to actually look at the jets and see what they are to get a starting point. They may not be stock jets.+100 I'm with JimmyD. I'd also suspect oil as the culprit if it indeed has factory jets installed. What mechanic suggested "adjusting" the jets? Unless thats his term for changing the jets I'm lost. Most bikes are set to run lean from the factory to comply with EPA emissions requirements and installing the aftermarket pipe normally makes it just a shade leaner. I don't see how the bike is running too rich with this unless you have severely restricted air flow on the intake.
A jet kit is probably a good idea as you will get a selection of jets and save $$ from having to keep ordering more jets.
I think a rough ball park starting point for adding a full race header would be to go one size bigger on the pilot jet and one or two sizes larger on the main. See what that does and adjust from there.
If you are actually running rich (and not oil fouling) it sounds like somebody already installed some big jets and maybe overshot the mark. If it's oil, you need new valve guide seals.