Hack on a /5
I put a Velorex on a /6 R90 and had a barrel of fun. No frame reinforcement but it wasn't ever loaded too heavy. A heavy chunk of flat stock steel to give the two lower mounts something to mount to is a good idea. To change gearing on a shaft drive bike you can switch out the rear ring and pinion or find a complete rear drive unit from a smaller displacement bike that will be geared for more RPM for a given road speed. Then if your speedo is off just install one of the bicycle type speedos or a GPS.