Those are ancient cord plugs and are all but unobtainium. You'd be better off getting a new cord block from Sew-Classic {
http://www.shop.sew-classic.com/ } and rewiring the motor and light for regular two prong AC plugs. The reason your motor won't slow down is that it's not being controlled by the foot controller. It's getting full voltage.
If you don't have a foot controller, Sew-Classic has those too.
Joe