I assume it is a direct drive motor (motor directly attached to blower wheel). I know Grainger carries a wide selection of blower motors, but from what I have seen, they have the motor (by itself) without the blower connected.
Anyway, you are probably looking for something in the 1/2 HP - 3/4 HP range. If you look at your current motor, what specs are on the label?
I can check my blower motor for you. It is an older one, but my electric furnance is also ~80k BTU and the speed is set to med-high for heat and high for AC (I believe).
You probably are looking at something in the 1200 CFM range for flow. With a multi-speed (or variable speed) you can slow it down to get the delta-T across the furnance that you are shooting for.