I have used easy-off oven cleaner with very good luck.
It is a pretty common trick with ysr50 racers(stock class)
It may not unplug your exhaust but it will remove oil,soot,and
carbon from the inside of the pipe.
cap one end of pipe and spray entire can of oven cleaner into
pipe turn pipe to coat entire inside and let sit for a couple
hours,then hose it out.