I would start by looking for the motor specifics on the net, maybe poke around in the Grainger catalog.
They may just tell you what you need and source it for you.

This sounds like what you have but Gast does not make the motor so You are going to have to sort that out somewhere else.
http://www.gastmfg.com/product_detail.aspx?ProductID=215&ProductTypeID=25


Greg Fretwell