I was told that rolling up some chicken-fencing and jamming it in the pipes should work but I've never tried it. At least it's a cheap experiment! 

Years ago on my shovel I jammed steel wool up the pipes and that got them through but things were a little easier back then...

Some small mesh chicken wire, folded into a ball or cone shape, stuffed with fluffed up stainless scourer pad and pushed up the muffler. about 100mm. Remember to leave some mesh hanging out, so you can hook a wire coat hanger to pull it back out. Another option is the stainless crimsafe mesh cut up into baffles(rectangular V shaped) and wedged in with a wad of scourer in front. You may have to remove the mufflers to push them back out though, using a bit of reo or water pipe. Have done this a couple of times when required and works well. Bike did run like shit though....but was quiet....