The cracks in the brass may be stopped by drilling a small hole at
the end of the crack, then silver soldering up the crack.  The hole
intercepts the crack and prevents further cracking, and is perhaps the
most exacting part of the process, as sometimes it is difficult to
find the exact end of the crack.

The appearance of the repairs can be improved by dressing down and
polishing the repaired areas.  It seems as if it would be necessary to
finish the inside as well so as to not cause the tuning slug to bind
and cause further damage.

Your air filter may be a shop vacuum filter.  Try at one of the big
box hardware stores.  A cursory Internet search turned up a Shop-Vac
air filter that's fairly close to what you have.  It could also be an
industrial air compressor intake filter.  I would think the builder
would have used a vacuum cleaner filter as they are generally rated
for higher CFM than equivalently-sized compressor intake filters.

Dave Vincent