First of all im dealing with a SOHC 4G63, but im not talking anything engine specific here.
Right now im running it in my 92 galant with just less than 200,000KM on it. It has the original O2 sensor so im pretty sure thats the problem. Unfortunatly im told that there are NO MORE O2 SENSORS AVAILABLE

Plus when they were available they were $450, and no I cant exactly get one from a junkyard because the car is fairly rare in canada. Its different than the DOHC so im stuck, and the dsmtalk folks havent helped. Im obviously in canada right now but I think the dealerships parts systems are based on north america as a whole.
Also I hear the "rotten egg" smell comes from fuel making its way into the cat? Is this true?
Could an AFC possibly be used to regulate the o2 readings? Would that be a good idea even if it could?
My car is notorious for bad fuel economy but will run on monkey piss with its 8.5:1 compression. Should I run a compression test maybe even if it runs fine?
Im selling it late april but I dont want to be selling a car thats going to run the guy bankrupt through the cost of gas.
Also, thanks again boost_boy for your help awhile ago on the SOHC performance options

ps. my replacement car is going to be an s14a so im looking forward to the transition.
Thanks in advance folks!!