I have blocked the boost bypass on several engines, to get rid of a power surge. The last 2 were a Magnum 36XLS (latest version) and a Magnum 28XL. The 36 is in a ARF Nobler. This engine has been used (stock, I believe) by Phil Granderson to power a much larger airplane. I tried 10x4 and 11x4 props with 5% and 10% fuel (20% oil). When the engine would speed up during manuevers, it tended to surge suddenly, gaining maybe another 1000 rpm. It would come back but the surge was not desireable for a nice pattern. I blocked the boost bypass and the surge went away. Further tuning with head shims (+20 thou), venturi size and muffler outlet, made the engine very consistent.
The Magnum 28 is on an ARF Primary Force. When I initially experienced the same problem, I just duplicated the work from the 36 and I've got a nice running stunt engine. BTW, both run in a rich 2-stroke. The 36 swings a 11x4, the 28 a 10x4.
To block the boost bypass, I disassemble the engine; clean the crankcase in hot antifreeze (crockpot); flush and dry thoroughly; scuff the bypass wall in the case with a Dremel tool; and fill the bypass with JB Weld (sufficient to block it). Try to get the level close before it cures, though excess can be carefully ground off after curing.
Fred