My custom exhaust uses a 7/8 pipe and it is sized very well.
Here's a little tip if you need a new exhaust gasket: Get an intake manifold heat riser gasket for an old VW beetle. The parts store near my house has these for $2.00. It fits very well and is much higher quality than the kit gaskets.
The two-stroke and four-stroke aftermarket pipes that we sell have the same diameter and mounting flange, but the bends are different. This is because the two-strokes have an angled exhaust port (angled toward the Left side if you are on the seat) while the four-stroke does not.