Source for an eta movement? Good question! In the old days you could just order one from Jules Borel or Ofrei for $79 but those days are long gone. I paid $95 for an Asian 2824-2 a few years ago. When I built my 1016 Explorer with a DJ 1601 case (same as the 1603), I used a 2824-2 and it worked fine. I used a Stilty spacer ring, but since you can't get those anymore, try a rafflestime #2 ring. Case clamps and screws you just need to scrounge up, maybe from an old watch. You might be able to find an eta 2824-2 used on ebay for a decent price and then service it for another $75 or so. Old watchers like Sandoz, Octo, and Zodiac used eta movements.
Just thought of one other thing- if you use a pie pan dial, you might be able to use a 2836-2 movement since the dial lets the movement sit higher in the case