Thinking of this on mine also, so far I've come across 2 off the shelf choices. Milltek and THS
The milltek is more expensive and the end is left at 63.5mm which won't fit your oe cat back. They want you to buy their complete system
The THS comes with adaptors to fit your oe cat back or another brand 63.5 system
The oe cat back systems o/d on mine is 55mm
You can get a reducer made to fit the milltek but that adds even more expense. Think i'm gonna try the THS, If I can find one

Good luck