You can also do a "quick and dirty" conversion by tweaking a few things.
a) halve any + anything bonus due to magic.
b) reduce all spell durations by one "tier" - 1 day becomes 8 hours becomes 1 turn/level becomes 1 round/level.
c) stat buffs do a flat 4 point increase
d) Polymorph is still broken; (imo) it shoud be replaced by the "alternate form" ability in the MM.