Still working on this. I got the colored buttons and coin icon working. The colored buttons look pretty much just like the mockup, so I won't post a screenshot, but I made a slightly larger custom graphic for the coin.

I've done the Action mode HUD and some of the background graphics.






I also did this little bit of text on the map screen.

What it really says is something like "destination", but I can't make that fit. As you can see, I've gone with "goal" for now. I'm torn between that and "target".
I've been researching the event scripting a lot. The whole game is pretty much run off this event script code that consists of a command byte followed by a varying number of parameter bytes. There are all kinds of commands. Various kinds of jump commands, commands for loading compressed or uncompressed tile maps, compressed or uncompressed graphics, loading palettes, copying/writing/adding to/subtracting from/comparing RAM values, branching based on the value at a RAM address, loading/writing text strings... There's a lot of stuff you could potentially do with it. Unfortunately the event script is restricted to one bank, and there's not much free space there. I was able to modify it to add a little quality of life feature that I had wanted for a while though.
In the unmodified game once you select "Quest" or "Action" mode on the title screen the menu will change to a 1-Player/2-Player select menu. If you accidentally picked the wrong mode, you can't back up. You have to just reset the game. I modified the relevant event script data so it now allows you to back out to the Quest/Action menu again.
I did some more digging into the debug menu. It turns out the code for accessing it from the pause menu is still there, but it's coded as a fifth menu item, and the RAM value for the number of menu items is set to four so it's never selectable. Changing that value to five makes it accessible and, for reasons I have yet to figure out, accessing the debug menu that way actually fixes the stage select. I also tested the "ENE RESET" option, and it does exactly what I thought it would do (makes defeated enemies respawn).
Click below for an unimportant but interesting (to me at least) tangent about old video game magazines:
Spoiler: I was curious about the EGM2 issue (I said it was EGM before, but it was actually EGM2) mentioned on the Shounen Ninja Sasuke wikipedia page, so I tracked it down. It turns out the article in question had nothing to do with an English release. It's an article from a section specifically about new games coming out in Japan. You can see the pages here:
Page 1,
Page 2It doesn't seem like they were very impressed with the game, but I also don't think they actually played it. I think somebody sent them a VHS tape preview. You can see the VCR UI text at the top of one of the screenshots. The issue is from August 1994, and despite them claiming the game was coming out that month it wasn't actually released until October.
The only things of interest to me really are that the screens are from a slightly earlier build of the game (the "lives" icons are gray squares with the characters faces instead of the 1p/2p icons from the final), and that the writer couldn't decide if player two's name was "Chin-Nen" or "Chin-Nan". More on that in a minute.
In the process of looking for that EGM2 issue I actually did find an issue of EGM proper that mentioned the game. This one I find much more interesting.

It's from the June 1994 issue, so a couple months earlier. It looks like EGM and EGM2 couldn't agree on the proper translation for the title either, but I don't really care about that. Look at that screenshot! I guess it's the Action mode HUD, but it's totally different from the final game. Vitality! N-Magic! Why the heck is Sasuke called "M-Buster"!? And player two's name gets yet another spelling!
Speaking of Player two's name, none of the above spellings are actually in the final release. The only official romanization is "CHINNEN". It pops up next to his health bar in Quest mode when he's getting his ass kicked.

Anyway, like I said none of this is really important. I just find pre-release stuff like this fascinating and I figured I'm probably not the only one so I thought I'd share my findings.