This is a chain-drive bike with the Nexus hub. Is it possible to convert to a mid-drive? I realize the chain needs to remain parallel to the frame and a friend who has built a few ebikes for himself says it won't work for this bike. He says I need a measurement of very near 51 mm from mid-frame to the chain ring and my bike measures 44 mm. Don't the conversion-kit makers have chain rings to adapt to bikes such as mine? Or is there another reason why a mid-mount motor can't be used with the setup?