Did I mention exhaust, not sure, but is always my first thought when a twostroke won't rev out cleanly & nothing else has been played with that could cause it.

yes any carbon build up in the exhaust system will severly restrict exhaust gasses.
my way is to wash it through with petrol and blowtorch the entire system a bit medievel but always worked for me

Here's an extreme case on my LS2 100 twin, both pipes, one worse than other though.
photos.app.goo.gl/NhaTErKf9DzfiJXEA
Here's your spark arrestor deep in most big twostrokes.
Credit for the pic goes to someone on the forum years ago by memory.
photos.app.goo.gl/gMNef9A3rxUAxgv96
Here's what i do in when neccesary haha.
photos.app.goo.gl/84UzSjyEzaRGwmxcA

Fire and loads of heat is your best friend on two stroke dirt bikes burn the crap out of it .
The Neighbours will think your house is on fire with all the smoke.
So i take it there is a spark arrestor in the silencer on this 77 DT400MX USA