Welcome to the board. Nice looking bike you have there.
Make sure you check out the "stickeys" in the First Generation Ascot Tech Section. You should change your jetting to richen up the mix & then change your countershaft sprocket to a 16 tooth. That is a nice little boost.
As far as the exhaust. There is only one after-market performance header out there. It is offered by ThumperStuff. Then you need to decide upon the muffler/tail pipe section of the exhaust. The least expensive that I'm aware of are the reverse megaphones offered by EMGO. Next up would probably be a Sports-Bike cannister type of muffler. If you have the extra cash, then look at Super-Trapp. If your bike is all stock & you don't plan to modify the engine, you can get away with retaining the stock head-pipes (if they are in good condition) & just install a different muffler. That's what I'm running on one of my Ascots.
These are fun & fairly simple bikes. They respond well to minor modifications & are easy to work on.
Good luck
J.