Congratulations! It does make so much difference to have the AGM battery and the Iridium plugs!
The backfiring on deceleration isn't really something to fix, since that's the way it comes from the factory, but if you want to try to eliminate it, go ahead!
Just gotta plug the hoses going to the reed valves (marbles do a great job) if you want to keep it looking original and it's also very cheap (just find 2 marbles to stick in there) and you can reverse it if you need to have it inspected for emissions.
Other way to go is buy coasters.
I marbled mine but it's still popping, exhaust gaskets are likely leaking. Doubt they've ever been replaced since the bike entered Mexico. I got bigger fish to fry for the moment though.