Damaged metal gears is not that common, it can happen, usually it's fine cracks developing over time, so called metal fatigue. Gears might need replacement, rubber belts usually needs to be replaced sooner or later and expected within the lifetime of a machine. If it's a very nice model, it can be worth the repair (I never was able to identify you 8 stitch model), but if there are better built models available, I agree, rather dedicate your efforts on one of them. Some vintage machines are fanstatic that way, after a fix up, they will service you for life.
I googled Kenomore 12, but couldn't find anything definitive. Does it look like
this one? If so, it looks like it's well worth spending a bit of time and effort on, even a service :- )