Oh, it will affect the bike alright. But mostly for the better.
Bigger exhaust means your bike can breathe a little better and hence can rev a little faster due to lower back pressure. It'll also give some boost to the torque, but it'll be minimal, next to unnoticeable on such a small displacement engine.
And yes, the fuel consumption will slightly increase as a result, but I think it's a liveable tradeoff.
You may want to buy a 'performance' air filter as well and change the carb settings as it's best to run the bike a bit 'richer' with an aftermarket exhaust.
