how much it still needs? a lot! 
Basically everything you see, is done, what's left is the rest, like hmm..., music/sfx, lots of rooms remaining (including "the gravitron"), map screen, CPU controlled players, dialog, collectibles (called trinkets), teleports, text terminals, titlescreen, ending, anything else i forgot to include.
Those two hard-to-get trinkets are already playable, without the actual trinkets. 
The hardest room is usually "Doing Things the Hard Way".
The things I'm most 'afraid' of are sfx/music, and getting the dialog script right. CPU controlled chars shouldn't be a big problem.
BTW even though this topic is not in the Development subforum, I'm fine with dev talk in here. The game is in *gasp* SCREEN 1! That alone should be able to cause a mad discussion between PingPong and hit9918. 
I'm scared of wolf, he would do newly composed wolfmusic for it instead of arranging the original ones. I'm sure it would sound really good, but nothing beats getting it close to the original version. 
I plan to use the replayer of MJTT and Dr. Pill. I'm confident I can do it, it would just take some practice (been a while since I arranged music )
BTW even though this topic is not in the Development subforum, I'm fine with dev talk in here. The game is in *gasp* SCREEN 1! That alone should be able to cause a mad discussion between PingPong and hit9918.
is there something to discuss? nothing. It's one of the few games that can be done without to much efforts on msx1. Even screen 1 with crappy msx sprites can do this. Probably there is no need of sprite flickering routine.
Me and hit9918 discuss only of interesting tech aspects of msx hw, not of every s.h.i.t. So, at least for me, there is nothing to add.