Someone put a welsh plug style heater in the water jacket cover (behind the carb) on my M. It has threads and they drilled a hole and bonded the nut to the cover. The heater screws into that. If you give it a couple of hours it does an outstanding job. I want to say it is a 400 watt, but it puts the heat exactly where it is needed not in a hose a foot from the engine. I am very impressed with the way it works.
Google it. It should be the same size as a "freeze plug" heater but have threads. Find a nut that will thread on it and put that on the cover. It isn't welded on so it must be epoxied or something. It is the slickest thing I have seen and works well. If it were me I would weld the nut to the cover although it would be tough because that metal would be easy to burn through.
I would have looked at it closer the last time I had the cover off, but I was afraid to mess with it much. If it ain't broke.