Don't know if you're still considering this bike, but it could be something simple.
If whoever installed the new exhaust didn't deal with the exhaust valve system properly, it would do exactly what you describe. The simplest way is to remove the cable, but keep the actuator on the bike and plugged in. It will still cycle when you turn the bike on and tell the computer everything is O.K. that way. If the actuator is gone but the plug is sitting there, you have probably found the issue.
Something to look into.