You shouldn't need a larger exhaust as long as yours isn't restricted. Having said that, you might like the sound of a performance exhaust.
If your friend did things right when he installed the BBK, he should have already gotten the jetting right.
Rollers and clutch springs are really a personal preference item. With the BBK on a four stroke, it should have lots of torque, so you might want stock or heavier weights.
How's it riding for you?
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