What is the OD of a 1" threadless fork, and a 1" thread fork? Are they the same? Could a threaded fork with a longer steerer tube from a larger frame be converted to threadless by sawing off the threaded portion?

Seems to me that if the OP wanted to keep his bike as original as possible (with steel steerer), the solution would be to locate the fork from a bike similar to his that has a longer steerer tube, and convert it threadless (by sawing off the threaded part of the steerer), and buy a new 1" threadless headset.

You could also try an alloy fork. 1990s Trek composite bikes (or a damaged frame with fork) came with 1" threadless alloy fork. This could provide you with the fork and a threadless headset.