For fast play of monsters in the 3.5E D&D rules, there are some quick conversions you can do on the spot.
Skills and special modifiers convert 1-to-1, and saving throws equal Fortitude, Agility, and Willpower. Ability scores convert easily by subtracting 10 and dividing by 2.
Hit points are generally multiplied by 1.5, although higher level monsters may have their hit points doubled.
You can keep the Base Attack Bonus (Strike) and Armor Class (Defense) the same, but you should add damage about equal to the monster's CR-1 with every attack.
All feats and special abilities can be kept essentially the same. Attacks should be assigned a speed class if they are not made with a weapon, and this can be based on the type of attack (mental attacks will usually be Swift or Fast, natural weapons will usually be Fast through Slow, based on monster size).