It does not matter if the engine's muffler has the baffle in it or not. The baffel helps to reduce noise and also a side effect is that it might increase muffler pressure a little more too. Unfortunately mufflers tend to restrict the engine's power  more and a baffle will restrict it even more too.

If the engine ran fine on the test stand, then you need to check the fuel tank and fuel lines going to the engine. I would suspect the fuel line inside of the fuel tank has problem. Such as the clunk is stuck forward in the tank and not sitting freely at the rear like it should. The other case is the fuel line has a hole in it, likely where it goes on the tube passing through the stopper. In either case when the plane starts to take off the fuel line gets air in it and it causes the engine to quit.

I would suggest only putting about 1/4 tank if fuel in the tank and adjust the engine for that fuel level. Then Point the nose up at full throttle and do your final adjustment there, set just a little on the rich side. Then stop the engine and fill the fuel tank up. Start the engine and you should be in business then.