All you need is a speedo from the last cop i spoke to. This being said, just have your phone mounted on your bike that display the speed via gps(?)
Odometer is to record vehicle mileage, which must always be on the vehicle. GPS mounted speedometer is fine.
All you need is a speedo, I was pulled over on my old hardtail and the cop wanted to know where my speedometer was...I had just a small 2.5" speedo mounted right near my right knee and that was good enough.