How is your present light attached? If there is none, pick your new one, take it to a machine shop and have a mounting plate made to fit with appropriate mounting holes for the light and bracket for the mount.
The original plate was probably knocked off by accident. On my older boat the plate was welded to the push pit.
Alternatively, you can buy a Magma Grill mounting bracket that attaches to a 1" rail and the bracket flange will hold the light.