i too tried a lawnmower muffler at first and all it seemed to do was collect oil and drip it all over the place.
i also was going to try to use a baffle from an 80c.c. honda dirt bike, but it was way too big to fit on my bike.
That's another solution if you have the room. try to find an older baffle from like an 80 c.c. or 125 c.c. 2 stroke dirt bike. look for an older one (like from the mid-late 70's) because they are smaller than the newer ones are. you can find them failry cheap on e-bay from time to time. the one i have is from an early 90's honda cr 80, but it's way too big for my bike and it's rectangular rather than tubular. i have a 20" frame and my bike is a lowrider, so don't have a lot of ground clearance or frame clearance. if you want to give it a shot, i will sell it to you cheap or trade you something for it if you want...i have no use for it.
here's what my modified stock muffler looks like on the s.t. chamber.
I hide the end of the muffler that has the small outlet hole in it, up inside the muffler tube, so you can't see it. with the slash cut on the end, it looks like it's just an open pipe.
If your stock muffler is not welded together, you can take it apart and modify it like i did. if it isn;t welded together, there will be 3 screws on the outside holding the whole thing together. once you get the screws out, you will have to put a bar or something in the end that comes from the engine (you will have to cut the pipe off for it to fit on the expansion chamber anyway) and pound on the bar to knock the guts out. you will have to put the bar against the peice (see pic below) and pound it, but pound it carefully. it is a very tight fit, but it will come apart. you will probably find that there is a peice inside the stock muffler that looks like this:
take this peice out and leave it out. it's supposed to be a cateletic converter of sorts for emissions, but in reality all it does is block the flow of exhaust collect oil and clog up the muffler over time.
if you want to try and do this, let me know..i can walk you through it more of you want. if you look close at the pic. you can see where my bar was at when i was pounding the muffler guts out of it.
