If you still have a cat, it may be clogged up (mine is starting to stink)
Cat is not an issue. exhaust is so loose that if it was clogged it'd leak from the loose connection from in front.
Noted in other places that I took a Vacuum gauge to the 2100 and tried to adjust it. Says it's running better. and was able to lower the Idle after the change. Still seem to be losing power under load. Going to like at a timing advance issue. Maybe even got a lifter that is not working well on the thinner oil causing problems after it warms up..
But I'm at the point I'm going to put 10w40 in it and look at timing..the Inline filter I put before the pump is full of crud.. But the filter after pump looks good.. May of sucked something in before but not sure..
I'm rambling.. Loss of power started after the oil change to the 5w30.. Change that to heavier oil, even out the adjuster screws on the 2100 and check timing..
Old motor having blow by issues and such maybe I'm trying to rationalize too much..
But all this work has allowed me to see I need to drop the tank and clean it at some point after I look at the frame.