The port runner is a straight shot to the throat area.just take a reamer and ream the port runner,the area directly under valve head doesn't need any work.Technically the runner should be tapered,its easier to continue with straight sided port wall.It can be tapered with a tapered reamer but you need to grind the tip of it back to get the opening at the intersection the diameter you want.
I have just stuck with straight ports until I arrive at a diameter that does what I need.
On the GX31 the port starts out at .398 I just picked the next 64th size reamer up from there and opened port in 1/64th increments,testing in between.
I'll take some pics.I have a sketch of the GX31 intake port that will clarify some of this.
The hard part is breaking the sharp edge at the intersection of the runner and throat/pocket. I was able to take a cheap needle file heat it up and bend it at the tip,rifler files would do the job very well also as they are bent.
It takes a little patience, but basically you message the edge with the file.All the work is done from the carburetor side of port so its accessible, tight but you can get in it. A grinder can be used but very carefully,you want to round edge evenly around the intersection,its easy for the tool to drop the bottom of the port floor and groove the edge.

Before anything of significance can be done with a cam this simple modification needs to be done first.By itself opening the intake port helps,you won't see a monstrous gain but roughly a 12% increase in flow will be noticeable.


EDIT:Added the following

I have some pics and a drawing I'm preparing,there are some issues on the GX25 that need close attention to avoid daylighting or breaking out of port runner.

BBriBro Did you get my PM?