I haven't thought much about the score and items and I think in worst case they could be displayed outside the active play area. I don't think the colors will be an issue for them either though but maybe having to blit them aswell every frame might be a bit overkill.
I did a quick mockup of the forest level, it looks a bit more brownish than the original which is a bit more gray, this could probably be improved upon. I haven't ripped any enemies from this stage so I just added some level 1 enemies to get some more life to the picture. The background is very sloppily drawn by me just to get a feel for it, I was thinking that on this level it would probably be enough with 3 color sprites for the background so it could be 128 pixels wide instead of 64.
http://driar.se/rastanmockup/rastan4.png