First of all, some definitions: I consider the classic Fallout games to be the ones currently available on this site (1, 2, and Tactics), and the modern Fallout games to be the more recent ones (3, New Vegas, 4, likely any Fallout released in the future). I consider this to be a reasonable dividing point, seeing as the gameplay changed completely to the point where it could be said to have switched genres.

Anyway, my question is whether there exist any mods that attempt to either:
1. Recreate one of the classic Fallouts in the engine of one of the modern ones, or
2. Recreate one of the modern Fallouts in the engine of one of the classic ones.

If such a conversion mod exists, how well does it work? Are there any parts of the game that don't translate well (and hence had to be rewritten), or does the translation work remarkably well?

There's at least one project I know of that is attempting (I think) to make Fallout 1 in New Vegas's engine. I don't know if it died though.

Personally, I think these mods are doomed. It would be impossible to translate Fallout 1 or 2 to the 3D games.

I don't know of any de-makes either. Again, I don't think it would work out too well anyway.

The original Fallouts (mechanics wise) are more about having X Ys blocking your path, how do you handle that situation. Enemies are specific problems. They will always be the same and their loadouts are always the same. They are there for a reason. Theoretically, the way they are laid out on each map is the puzzle to solve.
Also every map is pretty self contained. Fast Travel is the only way to go anywhere.

The modern Fallouts are about exploring one big map. Enemies are there but they're less of a puzzle to solve and more of a time waster that's in no real danger of killing you. Enemies are there out of some sort of obligation. Enemies are totally random for the most part and the real time shooting combat system of the modern Fallouts takes all the strategy out of the game. VATS is a further detriment. It's just a super power. What, does the game expect me to stand still and wait my turn while my AP recharges? Of course not, that's stupid.

So yeah, I don't think any projects like that will work too well.