Hi,
We don't have much information available for the Tornado H920 or H920+, it's not to difficult to Replace a Motor Arm, you'll need to Remove the 17- 2mm Pan Head Allen Screws from the Bottom Shell to allow removal of the Top Shell. The most difficult part is trying to squeeze your Hand in between the Top & Bottom Shells to Unplug the Motor Wires on the Broken Arm from either the Yellow Plug, or the Bullet Connectors on the ESC Board, as the used either way of Connecting the Motor on both the H920 & H920+.