Older systems didn't have the capacity for storing the entire SJIS code page. I've only seen one SNES game with anything even resembling SJIS, let alone an NES game. Every game had its own custom character set with the letters it would use.
I made a rough table. It is missing some control codes and such, but I think all the letters are there (and a bunch of symbols..)http://pastebin.com/bxk7Lxsd
You can use trial & error to try to pin down the missing codes or wait for me to do it tomorrow.
The game's opening text is in the ROM at $30D70
Are you trying to dump the script? Can you find each section of text in the ROM?