Engines do not cause crashes. Unprepared pilots do.

Bad mouthing an engine that has been used with carbs or mufflers different than that provided by the manufacturer is bad form and is not fair to the manufacturer of either device.

I would purchase an Ugly Stick type of model that is capable of utilizing your engines comfortably and then test fly them with their stock mufflers. This would give you an opportunity to become familiar with the engines under better controlled circumstances. Only then will you be able to determine if the engines are truly having difficulty.

Insufficient cooling air entrance or exit can cause an engine to overheat and quit.

Insufficient vibration isolation for the fuel tank can cause an engine's mixture to vary so much that it will quit.

It sounds as though you have invested considerable effort and thought into solving your problem. Don't give up yet. Try to isolate the problem by narrowing the possibilities. Good luck.