Use at least 3/16", 1/4 would be better.
I made a mounting plate myself by drilling two holes, then using a jigsaw and cutting out the extra steel to make a slot, then made 3 others.
Of course, if you have access to a fabrication shop, it's real easy using a hydraulic punch.
a portable massager
i would say use thicker metal and a sheet of rubber betweeon then engine and mounting plate, even just some gasket material would cut down vibration tons
i did it on my go kart:]
Did you fix it now?