ALL of the Wren 54's do that, there is not a machined stop. The trick they gave me was take a small strip of thin aluminum sheet and fold it in two. Place this around the front bow of the mount and mark (then drill) a hole where one of the cowl screws is. Screw this screw through the strip and the motor/mount-bow are now secure. Been using this trick on two 54's for 5+ years.
Greg