Then you just need a 90V transformer, a bridge rectifier and a lamp dimmer.
My local supplier sells transformers with twin 45V secondaries which can be connected in series to make 90V.
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Connect the bridge rectifier to the transformer's secondary and the motor to the DC side of the rectifier Do not use a filter capacitor, other than a tiny RF supression capacitor (<1µF). Connect the lamp dimmer in series with the transformer. Make sure the lamp dimmer is compatabile with transformers (i.e. is suitable for low voltage halogen lighting) and has a high enough power rating.
I'll post a schematic, if you're not sure.