If you take the end-cap off the Kawasaki KX450F's exhaust system and pull the core out from inside the muffler packing. Notice how few holes are in it?
No wonder the packing can't do its job! Simply drill a series of holes in the core, It's best to start with something like 4mm holes and follow-though with a larger drill-bit, and then reassemble the pipe.
You'll find it now sounds like a normal 450cc, and it didn't cost you 20,000 Baht worth of aftermarket pipe to fix it!
I have bought an 2008 Kawasaki KX450F and after a few hours' use, the thing sounds so loud and rattly in the exhaust department, you'll think it's demonic. Some local mechanics looked at it and they cannot find anything wrong with the engine....