If there are flash card things for the original GB I suppose one could check.
Good news: I'm 80% done. Replaced title screen, all terrain and character sprites, and fonts. It's a joy to play the game with one's own sprites.

UFO! Maybe the ladder should have a perspective shade so it matches the angle.
Water is animated. This is my frame but I'm having trouble finding where the second frame is at.
Missed the W and a whole buncha stuff. Animation is the hardest. Gonna have to boot up Arnimator...

I liked the spear fly so much it inspired the spider remake. Sausagy it wobbles down. Sheelcreepers now walking, but I'll have to tweak them a little still.
Problems:
1.) Wanted to replace the DAISY string with Pauline or POLLY or some such, since I drew Pauline. Can't find the strings even with relative byte value searches though. Even tried letter pairs since the strings might be 2x2 blocks. Perhaps searching for the names ending credits will yield insights into the string routine.
2.) The animated water tiles are forced into the tile table VRAM programmatically, it seems (index 221). I'd like to replace them. No clue where they come from though. Might be stored in a different format.
3.) The 1UP is a heart in this game. I didn't like that, so I turned it into a heart. This caused 2 problems. Hard to tell tell it apart from the regular shroom, and the sprite is also used as * pause * icon, and as a heart when Mario rescues Daisy. Solution: I turned the sprite into a pair of panties ._. Now, if only the game was about Pauline... I'd have to repixel the hardest characters though, the animated Marios... which were a pain to make and still aren't working.
4.) I wanted Pauline/Daisy to pilot the ship at the end, so I drew her in the cockpit. Mario is probably too tired to drive. The problem is that it looks like she's already in the cockpit when the couple approaches the ship... Might have to revert to my indistinguishable silhouette solution.
Edit:

Title screen. Very demo-fonty I suppose. Done no tile placement editing here. I'd populate the horizon better if I could. Placed some Moai heads there but they feel a bit even-tall (with the messy hills) and too densely packed. Cleaned up Mario a lot and was able to give him a shroom in homage to the SMB1 jp/eu cover. Doh! Missed a highlight pixel on the M.