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.
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.
