The steerer is just inserted and brazed into the crown. Any framebuilder can swap out a short steerer for a longer one. Or maybe even build you a fork. A local builder brazed a 30mm rake fork for my 62cm 3rensho for under $300. Longest threaded steerer ever.

Don't hack up a fork that will result in something both unsafe and of no value to anyone else.

Another thing to look at - there are still new road forks for sale with maybe 40mm rake and super long 1" threadless steerers. They're the most affordable option for folks with giant frame that originally used 1" threaded forks. Try Soma.