What does being told where and what something is ( https://datacrystal.romhacking.net/wiki/Super_Mario_Bros
. has much, and there is presumably source code to various level editors too) so you can poke it with a hex editor* have as any different to poking it with a premade level editor?
Edit. Also presumably a nice commented disassembly somewhere around ( https://gist.github.com/1wErt3r/4048722
beware, might slow the browser a bit until it is loaded).
*a generally poor tool for ROM hacking that most around here would not advise you use, and if they do it is because it is OK as a first pass for file analysis or to tell some other hacker how to do something simple or variable that you don't want to patches for each possibility for. Doing something with a hex editor is not really impressive, just a sign that you are a masochist or can't code/fiddle with even a spreadsheet.