If you have a h.t. 2 stroke, with the older style muffler (this is a muffler held together with screws). take the 3 screws out, and take the muffler apart. This will take some work because these mufflers are fit together very tightly. I used a long brass rod, inserd into the muffler against the baffle plate, and i hit it with a hammer to get it apart.
Inside you will find a catalyst, which is a peice of pipe about the same diameter as the muffler, and it has corrugated steel inside of it. Take this peice out and leave it out. I'm guessing that this peice is for emissions, but in reality it's a cork in the exhaust flow. This peice is located at the end of the muffler, closest to the motor and is held in the muffler with one of the screws through the side of the muffler body that I said to remove in the beginning.
when you get the muffler apart, you will find the baffle (a skinny pipe with random holes in it) and a baffle plate (welded to the skinny pipe with the holes in it). notice the baffle plate...look at is closely. you will find that there is ONE TINY hole in it for exhaust to get through. I was surprised wen i found this because i have no idea how exhaust even got through this hole. on my muffler, it looked like a hole made by burning through the plate while doing a very bad weld. the plate fits in the muffler body VERY tight, so exhaust can not flow around it...it will only flow through a hole(s) in the plate.
here's what i did...i drilled 3 small holes in the plate on the opposite side of the baffle that the exhaust exit is on the muffler cap. This way, the exhaust can flow, but it will have to go through the baffle before it exits the muffler. This keep the sound fairly close to stock, but allows the muffler to flow ALOT better. I'm running this muffler on the end of an expansion chamber, and it's only a little bit louder than stock, and it sounds very healthy.
If I were you, and you are concerned about exhaust flow...get an expansion chamber, and then modify your stock muffler to go on the end of the chamber. An expansion chamber will give you more power, and a better sound. You can even get an aftermarket stinger (muffler) to go on the expansion chamber so you don't have to go through all the trouble of modifying your stock muffler if you don't want to.
drilling a hole or holes in the muffler body itself will only increase the noise. It will not do much for the flow. My friend drilled holes in his muffler body. He started with one hole and drilled others after test riding it every time he drilled a hole. the only difference there was, was that it got louder. the engine did not gain any power.