Well the engine is the same dimensions just a higher compression so I am confident the belly of the pipe is the right diameter. I think first that the stinger is to small in diameter which is increasing pressure in the chamber and speeding up the exhaust waves.
I see two ways to fix this.
1. decrease pressure through a very slightly larger stinger.
2. retime the waves by increasing the length of the belly.
As has been suggested I think cleaing it out as best I can should be the first step. This is the original pipe and is certain to have carboned up inside. Cleaning it can only do good things, it is not destructive and only requires figuring out a way to do it.
The stinger is the simplest change, lop it off weld on a section of larger pipe. I can then experiment with differing restriction to get the pressure right before settling on a final end can.
The belly is more involved but I think offers the greatest flexibility for future tuning. My plan would be to cut the belly in half across the diameter and changing the length to retime the return wave.