What about using some footpeg brackets from a modern bike? Today's aluminum-framed bikes have steel peg brackets that bolt to the frame. You could modify some modern steel brackets and weld those to your frame with the added benefit of being able to use modern, large footpegs.
From the photo of your countershaft sprocket, it looks like your footpeg brackets bolt to your frame, too. That might make it even easier to adapt some modern pegs and brackets to your bike: modify the modern brackets so they simply bolt on in place of the old ones.