There was a recall a while back instructing the removal of all flame traps. So it's a good thing you are missing it as it probably means the previous owner's dealership removed it.

As for the crack, that can happen with all that heat and all those gasses. If it is vaccuum sensitive, I would have it replaced or repaired as soon as possible. Vaccuum leaks cause all sorts of problems such as premature O2 sensor failure, premature cat failure, as well as other emissions related problems.

The affects the crack may have had on the car is entirely dependent on whether it is vaccuum sensitive, if it is, you have probably been running rich, and allowing dirty, unfiltered air into the engine. Which if has occured, has lowered your fuel efficiency, decreased performance, caused increased engine oil contamination, caused increase cylinder wear, among other problems.

Regarding maintenance, replace or repair the unit or the crack, and think about an early oil change. As for other problems, not much you can do. What's done is done, and just replace when they fail.