If you don’t get sorted, it might be worth tracking down a set of cheapo 520s and simply buying them. Given the cost of cleats these days, it means you can buy the pedals for 20 quid or so, get a new set of cleats which retail for around a tenner or so and have a lot of spare bits – SPDs eventually wear an oversized notch in the front locating plate, which means the cleat will no longer stay put no matter how much you tighten them, so you’ll use the bits eventually.
It seems wrong in terms of sustainability, but economically it makes some sort of twisted sense.
Which bit is it btw and which pedal? I have some sacrificial 520s lying about somewhere and might be able to bung the part.