Wow! I knew that on some of the very small frames they went with a one piece head tube/lug item, Surprises me on a frame that size.
Hope it did not have the full suit Reynolds transfer... Or, Trek was emulating the French volume producers.
A new set of lugs and head tube Could be grafted on. I would be thinking retirement, careful measures as I think you noted you like the bike's geometry and create a new one.
Maybe take Doug Fattic's class? Yeah, you can braze, but his approach to avoid problems is sound.