you should have no trouble putting in any number of off-the-shelf VFDs, esp without rigid and without tool load compensation.
Not sure why a replacement inverter requires a new motor? I sure don't think that's right.
My VMC40 arrived with a dead inverter, was replaced with a newer model (Baldor), no change to the motor, works pretty well. Nowhere near $8k!!! (and covered by warranty, too)