I did this very thing with an old MTI Rotophaser, that someone gave to me.
the unit is basically the unpowered top half of a leslie. When I got it, it was in bad shape. I wired it up the way I thought it should be wired. I installed a 12v powersupply & a relay rated to handle the current draw of the motors. This I used to switch speeds without having to run 120 ac to the footswitch (sounded dangerous to me!) I used the same power supply to control the bypass relay. you just wire it to bypass the leslie and connect to the amp spkr when not in use. wiring a relay is basically the same as wiring a switch. there should be a diagram with or on the relay itself showing connections. the relays I used are rated @ 12vdc control voltage & 4 amps on the contacts. Its fairly easy if you plan it out before hand on paper.