Most (all?) of the posters here are more experienced than I am, but I believe you can modify the exhaust on these bikes and still ride them WITHOUT making changes to the A/F. The bike will not be optimized, might run lean, but unless you're consistently in the high RPM range (+9,000), the risk of inflicting damage to the relevant components is minimal.
The bottom line will always be a dyno tune with some form of A/F controller, as others have said here, but changing the exhaust system doesn't mean the bike must remain off until said dyno is performed.