go look for the original shape exhaust muffler for the G100, its not the G11 shape, its pretty flat. The centre muffler is longer than the G11 style too. When in good condition the original exhaust should sound a little bit under load.
If you cannot find a good quality one, I would suggest you get one handmade out of stainless steel sheet and make it chambered and padded too. the furthest chamber should have perforated pipes with some holes in both. That creates the burbly sound you want.