This is a diligent but routine piece of work.
The black bomb for example could replace the SMB3 bob-omb
the walking enemy:
npc-136
after being stomped:
npc-137
after the explosion:
effect-72
Depending on the diference of the graphics size you might need to change some settings, look here for details:
https://docs.codehaus.moe/#/features/npc-config
The black koopa, is actually the bomb-koopa from super mario land
Some cool dudes released a npc-pack including it, called
"[The Ultimate Super Mario Land 1 + 2 NPC Pack!]"
viewtopic.php?t=28422&start=20
Chain chomb is done by "MrDoubleA's NPC Pack v1.4":
viewtopic.php?t=25011
Angry sun "[NPC Pack] Super Mario Maker Pack"
by IAmPlayer
viewtopic.php?t=24994
Use this SMBX2 Custom Assets Index for more details:
https://docs.google.com/spreadsheets/d/ ... 1014877093
Everything else seems to be like I said: Replacing graphics, adjusting the frames, change gfxheight/width and stuff like that.
Just give it a try.