New crank, although you could try a helicoil first.
helicoil
That's gonna be hard to find in that size + left hand thread
I've got some somewhere in the workshop, they're made specifically for pedal threads so not too hard to find, might not be helicoil branded. It's a left crank though so probably easier to just replace it.
@kjlem started
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New crank, although you could try a helicoil first.