I've got a DG pipe on mine, and once the header clears the engine, it's almost a straight shot for the muffler.  It's been a long time, but I don't think there was much difference in size between the DG header and my stock header. 

I suspect you could find a universal one that would be fairly easy to weld in place.

About the wiseco piston, I have know idea if it's true or not.  I just remember reading that somewhere on the internet and wanted to give you another thing to check, if you still have the knock.  Let the engine warm up and see if it goes away.  A call to wiseco might clear that up as well.