I have a '67 Porsche 912. There's a big premium on matching numbers in early Porsches--I can't see why, but it makes a few thousand in the value. The whole thing is pretty silly, when you start thinking about what it means. You can replace everything except the one small piece of the engine case that has the number on it, and it's "original." Even if it has been significantly modified, with oversize cylinders and the like. Conversely, there have been cars that received a replacement engine very early in their lives--sometimes the serial number was restamped, sometimes not. So, one is original, and the other, with exactly the same history, is not. And, in the end, absolutely none of this is relevant to anything real about the car.
My own 912 has a dead stock engine but no serial number, indicating that the engine was rebuilt, at one point, with a factory engine case. I'm just as happy that way, not to have to deal with the "matching numbers" silliness.
I don't mean to criticize your desire for an original engine. But I would like to make the point that the car will be absolutely no different with a well tuned, nonoriginal power plant, and you should enjoy it just as much.