steff,

just swap the motor. on the older bridgeports you had either a bunch of pulleys and needed to swap belts or the variable pulley to change the spindle speed. either used a single phase motor run at 60hz. if you run a regular 3 phase motor with a vfd, you will lose torque at low rpms or at higher rpms if you over run it. if you run it at 60hz you don't need a vfd. buy a single phase motor at the correct rpm and be done with it. surpluscenter.com is usually where I get my motors. that or ebay.