variable speed blower

I believe you should check if this model is available with a separate motor and end casing. Not all are available as separate components which would then mean the diagnosis of one or the other is a moot point. You are now in Pro territory....
You said that the motor was OK but

Did the motor turn easily enough? Ball bearings OK?
Ohm each motor winding to the case for OL.
Olm each winding to each other. Should measure within 5 to 20 olms and within 10% of each other.
If all is OK, the motor is probably OK.

After unplugging the 120V motor supply and letting it drain for 5 minutes so you don't get shocked, unbolt the end casing.
Does the motor spin differently when the motor is plugged into the end casing as opposed to when its unplugged from it? If so the end casing has a short and is shot.
The only repairable part within that I know of would be the thermistor if it looks burnt, disconnected or swollen. That can be ordered from an electronics supply house or tracked down through Google and soldered in to replace the old one.. It will be a 1 olm 20 Amp or a 1 olm 30 amp depending on the size of the motor. The OEM's don't help here with thermistors. Beyond that, its ordering a new identical end housing if it's available, and for big $$$.