I'm not aware of any compatibility between aftermarket kits and the OEM one, so I think yes, you'd have to replace the entire thing. Interesting idea about aftermarket- they are very cheap, around £25 or so IIRC for the complete kit. I would think it might be a little tricky to get the sensors to fit in the holes but for the price it's worth a go.
One more thing, before they condemn the sensors, have they taken them apart and cleaned them- sometimes this works.