*disclaimer* i tried searching, no luck. Asking for a friend so specific details may not be available in post #1. DRZ 400 SM motor was replaced. After the swap it started fine, ran fine, sounded fine, felt fine, power delivery fine, etc. The concern is that the exhaust header changes color to cherry red. It is a single cylinder and seems to run ok so I wouldnt think spark or carb or compression trouble. It is the same exhaust (yosh) which had no trouble previously. Bad exhaust gasket or something wrong with the pipe?
Agreed with @rob linders that it's common. Is it glowing when sitting at idle in your garage? Perfectly normal. You need to get that thing moving and the glow will fade. If it's glowing red while you are riding it, then that's likely a jetting issue.
This ^^ We used to get calls from people when we were a Suzuki dealer, all freaked out because the pipe on their new 4 wheeler was glowing. Usually the first time they noticed it was at night. If it's jetted right it's perfectly normal.