Really not necessary to parallel a ground (common) for all of your electrical components, normally.
If the motor is mounted on any anti vibration rubber then you just need a bonding strap between the engine and the frame. Except for Motobecane (with thier unfortunate and very shytty placement of the bonding strap between upper engine mount and external coil) most manufacturers have the common (ground) situation well thought out. Moby cure is just to add a new bonding strap.
IF you have decided to use stainless steel fasteners on your bike you may find the shytty conductivity of stainless to be an issue with some of your electrics. Similar issues can occur if you repaint a bike and there is no metal contact patches made for electrical bonding.