I got a GX25 to examine and found a few interesting things I'll pass along.

The intake and exhaust valves have the head diameter as the units in the GX31 but with smaller stems(more flow for same head diameter with smaller stem)
The discharge area of the intake valve is greater than the port runner ( straight section immediately after carb)
The throat area (directly under vavle head)also has greater area than intake port,this suggests that port runner be opened to 10.5mm (.413") to improve flow.This would let the engine breath as much as the valve opening will allow with stock cam.
Ideally the smallest area in intake tract would be discharge area of valve,with the port leading to this somewhat larger to insure that port velocity is increasing as it flows towards valve.
With a .413 port the throat area under valve would still have a larger area than runner,so to maintain the decreasing port area as it gets closer to valve,the ideal port runner diameter in this case would be .453" (29/64") The next size up in carb would probably need to be used.IE 12.2 mm
The exhaust port is already in the correct configuration I wouldn't touch it,except for some blending (just round sharp edge not recontour port) of the inside corner at runner and throat intersection.Same blending for intake.
Stock port is .377" diameter,with a .256 restriction in intake, these intake restrictions serve to limit rpm as there is no mechanical or ignition rpm regulation.
They also limit power available.
Larger ports work better at higher rpm levels,and may not be suitable for lower rpm applications such as the range aircraft are using.
I am not suggesting using these modifications just sharing what I have found on the GX25.