I started with the rulebook, thinking, if I could get used to some common phrases -- like Barbarian, Wizard, spells, attacking -- I'd understand the quests a little better. It's slow-going so far, especially when the text isn't easily copy/paste-able. Here's what I've been working on for about an hour. It's the first two lines of the rule book.
Japanese copied/transcribed from scan
ヒーロークエスト
ルールブック
ある日、4人の勇者がニーフ島の王に呼び出された。
これがヒーロークエストの始まりで ある。
Google Translation
Hero Quest
Rule Book
One day, hero of four is called the king of Neef Island.
This is the beginning of Hero Quest.
My Best Guess
HeroQuest
Rule Book/Instruction Manual
One day, four heroes were summoned by the King of Neef Island.
This is the beginning of HeroQuest.
Not bad, huh?

EDIT: Pages 1 through 11 translations done. See the attached pdf.
EDIT 2: Deleted previous file. All pages complete except Index on page 23. See the attached pdf below.