last half of muffler glowing red after rebuild

[goat124]

I just rebuilt the top end of the motor with new cylinder and piston kit. i have been running short trips to make it break it self in and i just went on a few mile ride on it and it started to spit and sputter so i stopped and took the seat up and notice that the muffler was glowing red tords the rear about where the band goes around where the exhaust dumps out. do yall think the muffler is toast and plugged up? is there a way to clean it out that yall know of or any way of using a muffler set up for a 2pg motor because that muffler is like 200 bucks compared to almost 700 buck for the right one for mine. it will crank and run fine after it sits for a while after it cools off. any help is greatly appreciated.