RE: ys engine surging at full throttle
The liner still has the cross hatch honing marks. How much fuel has this engine seen? Did it ever run well in the plane it is in now?
Two other things to look at, first is the o-ring from the carb body to the air box.
Second would be cooling for the engine in the current plane. I wonder if it is getting hot enough that you are seeing a bit of fuel perculation.
Other than those thoughts a ring would be a cheap enough piece to replace to try.