I've restored several Austin Healey cars. There are enough cars and people working on them to support a healthy industry supplying replica parts, both mechanical and body parts.
There are some similarities with the warbird situation.
Every part is now available for these cars as a replica except engine blocks (I stand to be corrected, but I think that is the position).
However, I have heard that someone is thinking of casting new blocks, and I would say that is just a matter of time before they're available.
I know for sure that new blocks are being made for Vincent motorcycles.
I'm not an engineer, but I think that technology is on the side of the block reproducer here. Short run production of good quality parts, even complex castings, is probably becoming easier rather than harder.
Notwithstanding anything above, a structural engineer once said to me about buildings - you draw it, we'll make it work. All you need is money.
I'm sure that's right, and if one were absolutely desperate to have a new Merlin, and if cost were no object, one could be made immediately.
But as you imply, it will probably take a tightening of supply before someone takes the plunge.