Re: Looking for FH replacements in D&D Adventure...
Sure since you have the monsters minions and marauders book there is no problem.
AD&D Gnolls = FH Erqigdlit Yes
AD&D Kobold = FH ???????—There are so many different versions of Kobolds in dnd/adnd that, you have to use your own description and picture but for stats you can use ratlings who are very similar to the Kobold illustrated by the Tony Detressetti (spelling?)
AD&D DOPPLEGANGER = FH ??????? = Mausquer from the same book that has the Erqigdlit
AD&D Monstrous Spider = FH Giant Spider Yes
AD&D Hobgoblin = FH Hobgoblin Yes
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Remember to tweak the numbers if in doubt. Go easy on your players until you get a feel for how much they can take and still keep going.