You won’t get a tapered steerer into an inbred frame, sadly. I’d like to try one of the on one rigid forks in my 456 but they’re designed to replace forks with quite short travel - 456s are for long travel forks so not sure what it would do to the ride.
The tapered forks could take the wide moon bike wheel I think, so replacing the stem is a cheaper option than getting a custom forks made. But there are no more wheels for sale.
I have a set of one 26" rigid forks you could try, and see what it was like before spending any money.
Balloon tyres as suspension.
On one/planet x have the ballon front wheel for £20.
Was thinking the on one forks that are tapered might fit the inbred ;)