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I was in the same boat trying to find a straight steerer fork for an Inbred. I eventually got a rockshox revelation from 2009 but it was still pricey. I paid £180 for it. But it had a 20mm axle. The project became too much of a pain in the hole, abandoned the whole thing and sold it all on.
If you're not in a hurry I'd keep the inbred and keep your eyes peeled for a nice 29er hardtail complete.
Gotcha.
It's twice the price of the whole bike though (+ it'd need a new wheel) so this is where I go back to thinking I'd rather just buy a whole new/used hardtail and leave the Inbred as a fat-tyre pub bike. Or maybe I just chuck the knobbies on and use it as a SS gravel bike.