This would be my solution, 2 birds with one stone. Mount motor on 10mm plate which would act as spacer, with existing bolts, then where bolt heads are enlarge existing holes in original mounting plate. Heads would be flush with original plate mount, no need to countersink holes. As said by foz above bolt new plate to original plate with bolts moved out of way of belt.
Unless there's something I've not thought of, very likely :facepalm: , this moves drive pulley in 10mm and bolt heads on motor are out of way, belt doesn't need to move in relation to motor.
One final thought: is it possible to rotate the motor mounting angle to change the orientation of the bolts so that one (or better still two) is inside the loop of the belt and therefore not likely to touch the side of the belt?