A plumb bob will do to check alignment, but if you are having to move the motor platform so far that the tension lever falls off then something else is causing the misalignment.
One of the pulleys may have moved or been incorrectly fitted during overhaul.
If the motor has been changed & is a different frame size,this could also be causing the problem.
As a quick fix, a washer or two fitted to the tensioning lever stub shaft between the handle & the frame should do the trick.