You can actually unplug it indefinitely. The AIR pump only operates for the first 30 to 45 seconds until engine is properly warmed up and fuel being fully burned -- so harmful enviromental pollutants is limited. I don't think even the most environmentally sensitive on this board would throw a fuss. It is, however, illegal so act accordingly.
About rebuilding existing pump ... I'm pretty sure you'd get a core $$ return if you replaced it with a rebuilt one .. not sure how you'd rebuild it yourself. What problems are you having with it?
Check the 2 rubber elbow hoses that connect pump to head, for cracks/holes before you conclude the pump is bad. Sometimes oil will drip in this area and ruin a hose, leading to a muffled diesel-like sound which fools some that the actual pump is going/gone bad.
'91 300E, 213K miles