I have the xnova 4020-1350 in my 516.
Its a popular alternative to the scorpion 4020-1320.
I found it best to route the motor wires thought the slotted opening in the motor mount then connect to esc wires below the esc tray. I had no trouble tightening the motor mount screws. The attached pics should explain it for you.
I set main gear mesh by manually feeling for backlash between the main gear and pinion. Adjust motor so there is a very small amount of freeplay between the gears when you rock the mainshaft back and forth after tightening the motor mount.
Then progressively check the freeplay as you rotate the mainshaft. Lock it down when you're satisfied that there is minimum backlash without binding between the gears at any point.
Hope that helps
Sent from my SM-G930F using Tapatalk