Thanks for the replies (thus far). The current speed control is missing parts and is non-functional.
I've read a little bit about using an air gate, but with the motor already wired-up for a speed control. Is there an advantage to running an air gate?
Here's one option: http://www.grainger.com/Grainger/fan-speed-control/hvacr-and-appliance-controls/hvacr/ecatalog/N-guh?op=search&sst=subset
I'm just not sure which speed control I should choose: 3 AMPs or 6 AMPs?
Thanks for all of the help thus far. I'm off to the guy's house to examine the motor more closely.