Quite a few crown races are interchangeable (and available on E-Bay). So, you may be able to use either a 26.4 or a 27.0 crown race with your headset.

Note that 1" threadless stems are relatively rare.

I don't see why you couldn't chop off the threaded part of the fork, as long as a star nut will fit and hold. However, depending on the frame size, you'll have to have a very long steer tube. The bottom of the threads should be about 2"or more beyond the length of your headtube.