If it were me I'd buy a new Canon BC-02 cartridge. (The 500 page yield BC-02 replaced the 450 page yield BC-01.) They can be had without spending much; and if the printer works it sounds like you'll need one anyway. I skimmed the StyleWriter II Service Source manual, but I didn't see anything too useful. Visual inspection of the innards is worthwhile too, of course. If it isn't the cartridge, perhaps there's corrosion on the contacts where the cartridge mounts? Or the locking mechanism for the cartridge isn't tight?

Just a few thoughts...good luck!