Here are two screens of it:
http://s000.tinyupload.com/?file_id=033 ... 1410451061
I tried to keep it close to the original, but also changed some minor details, like having a few more different enemies, but also a slightly faster elevator at the end. To compensate for that, I made the elevator's shaft a bit longer, so the elevator takes only around 2 seconds less than in the original.
The elevator in the shadowrealm is different, as the player needs to hit a block every around 2 seconds to rise the elevator higher, and it goes down slowly if he dosn't hit the block. And to give that mechanic an actual sense, I made lava that rises slowly into the shaft, so the player needs to be quick with the block, while enemies are interrupting the player to push the block continuously.
Bugs that occured while designing the level, were that setting Boom Boom to "legacy boss" would not spawn the SMB3 winsphere anymore after beaten, as soon as I placed a second Boom Boom in the level, that shows a SMB3-star upon beating, instead of being a "legacy boss". Seems to be a bug in SMBX itself, as the "legacy boss"-flag worked before I had added the second Boom Boom in the shadowrealm.
I fixed this bug by using layers and events to let it look almost exactly like the "legacy boss"-flag is still working correctly.
The only small problems that persist are, that the blocks that fall from the roof into the lava, do not stay together like in the original level, so they do not look like one single tetris block while falling.
The other small issue is, that when the player hits the block that rises the elevator in the shadowrealm multiple times in a row (like jumping under it like crazy), it sometimes can be a bit hard to rise the elevator, because of the events interrupting the movement of each other, but I think this is not a real problem.
Please check the level out, so I know what to fix or change, in case that is needed.