As nice as Dicks transmission looks in the pictures it will roll back to a nice grey color after some time. It may take a year or more to do that but it will. I had a Magnesium 5 lug Gas Burner polished to a chrome look to see how it would age. After some time, it got a milky white coating on it. I decided that the other 4 wheels should just be painted in silver like the wheels from the factory.
There have been a lot of 914 articles on Mag and painting it.
I do know of a magnesium small fire that happened at Bieker Eng. in the late 70s but it was put out with the right fire extinguishers (Class D fire extinguishers are used on combustible metals. These include magnesium, sodium, aluminum, and titanium) This was from machining a Eng. case
In high school the teacher in Science burned some Mag ribbon. Super bright.
Use a small die grinder and take at easy but why bother. The wire wheel like Dick used will give you that look and that is much better than paint.
Eric restored my 914 trans on 41 with Walnut shells I think it Looked good and was realistic of what these old funky looking cases can look like. Sorry I do not have a picture handy
Bob B