Hickory is right. But you don't need to take my word for it. Go to pocketplane or gibberlings3 or something similar and download the editing program DLTCEP. Take a look through the opcodes and spells and effects and whatnot that are not in BG1.

As a case in point, with BG2 it is trivial for a modder to make an item that is usable x-times-per-day. The only way anyone has found to accomplish the same effect in BG1 is to summon a (tiny, invisible, "invulnerable" (not actually possible in BG1, so you use a workaround)) creature, have it cast the spell, increment a global variable, then destroy itself.

BG2 in the BG1 engine is called BG2. It's the same engine, with all the additions and tweaks to make BG2 the game that it is.
If it would be done backwards, you would have to do so much to get the BG2 content running, that you'll end up with BG2 or probably a worse version.

If you want BG1 style gameplay in BG2, that is actually possible; with tweak packs you can get slower walking speed back and lower xp for picking locks etc. if you prefer that. I don't know if there are mods to limit your class selection, but if you want less options: don't choose the kits and don't dual wield. There is a BG1-style interface somewhere, for those who prefer that one. I think someone even made a mod to replace the BG2 character models for BG1's.
Lots of things are possible already if you want the BG1 experience in BG2.

But reverse engineering BG2 in the BG1 engine like you propose is folly.

If such conversion was made, I might actually like BG2 to some extent. During the development of the sequel there were some screenshots that look closely to the BG1 engine, it's probably quite early stage. There is a mod (Infinity Animations) that can restore the original character animations, but dual-wielding will cancel them, though.

BG1 and BG2 versions of the Infinity engine diffar quite a bit, so it could be quite an interesting experience.