I've ported 3 GX heads over the years and simply used a dremel; nothing fancy clean up the casting marks, matched the manifolds and gaskets to the head. The one sport I removed a bit of metal was around the valve guide bowl area to smooth it out. The other area was the valve seat; where the seat meets the heat it protrudes into the head by as much as 1mm meaning it's cutting the diameter by 2mm, simply getting the seat flush with the port wall where they meet makes a big difference.

Yes an H89 oval port head or GX head would be a better way to go but since it's only your time nothing wrong with cleaning up the head you have. If,your budget is tight I would install larger valves and clean up the ports in the head you have. A good head may be anywhere from $500-750 so working with what you have isn't always a bad idea.

If your budget as room in it as the others have said just buy a better head.

Tom