You can indeed buy a Variable Frequency Drive and slow the motor down, however the existing motor is not inverter duty rated. Better off replacing the motor as well, but that introduces a new wrinkle: the motors are rated for their horsepower by being high speed and low torque; slowing an AC motor down means the motor has (the same) low torque at slow speeds. The sewing machine conversion motors on Ebay (last I looked) were still relatively puny for leather work IMHO.

I actually did convert my metal lathe to a VFD and a larger motor. I kept my pulley arrangement in place though so I could change the belts for slow speeds while keeping power (like cutting threads).

Had I kept my industrial sewing machine, I was going to make a jackshaft arrangement so I could use pulleys to slow the machine down, while keeping the original high speed clutch motor. Unfortunately my wife died and I lost all interest; I eventually sold off the industrial machine as well as most of my other sewing machine "collection". I did keep the 3 best ones though.