Flash, opopop uses OS 8.5 - the app you're recommending is OS X only. There is another app called Keyboard Korrector for the classic OS, but it sounds a bit dodgy - nobody at VersionTracker was particularly happy with it.
Best option - buy a new keyboard! You probably won't be able to fix the keyboard easily or cheaply. You can get a new keyboard for anywhere from $15 (eBay, if you're lucky with a bid) to $35 or so for a new MacAlly or similar. Just make sure that it has pre-OS X compatibility, as some of the new ones don't. CompUSA has various keyboards if you want to try them out. Of course, the newer Apple Pro Keyboard is much nicer than the older iMac keyboards, but it costs $59. I got one for my Sawtooth G4 and love it.