I have seen complete plans for homemade quilting frames on eBay. My guess is that you would want to build the entire frame because any new carriage may not fit the old frame, especially if the frame is made of wood and might have bowed or shifted shape somewhat over the years. Also, carriages are specific to frames, so you might not be able to find a carriage kit or plans that would actually fit your existing frame.
If you need a place for the foot pedal, it means you are planning on using a domestic machine on the frame. Machine heads made for frame quilting typically have on/off and speed controls on the handle bars. People do quilt on frames with domestic machines, so you might need to start a separate post asking about how they handle the foot control.