The mechanical variable speed drive performs some torque multiplication at low speeds, while a VFD is at best constant torque. To maintain low speed torque you will likely need a larger motor than original. Some inverter rated motors are capable of operation at considerably higher frequency than nominal (in constant HP, diminishing torque mode) So you may be able to gain some low speed torque via mechanical reduction and regain some of your high speed range by running the motor faster. If buying a new high performance motor anyway I'd go for a somewhat larger one. The speed range in direct drive was only about 5.5:1 so that doesn't sound too crazy . If for example, the motor was running at 120 Hz for maximum speed, then low might be around 20Hz then assuming drive and motor does maintain constant torque you'd have 1/3 the rated Hp. It looks like various size motors were original options, 1, 1-1/2 or 2 HP, I might consider a 3hp for a new motor, but a 2hp might give performance somewhat comparable to an OEM 1hp at low speed considering that the mechanical VS drive would not have been 100% efficient,