This youtuber has put a Yamaha Banshee 350 motor into a Honda Grom. The Banshee was a quad producing about 34 bhp (a standard Grom is about 10).
He has fitted expansion chambers and in this video there are no mufflers - you can plainly hear when it comes "on the pipe". It reminds me of the days I'd leave Cadwell with my ears ringing.
The riding starts at 5:57
https://www.youtube.com/watch?v=OKJmniH9oDc