The fan shroud on the 4-cam engines was made of cast aluminum (might have been Magnesium) and was quite light but really rugged. The engine shop I haunted when I was a kid had a whole engine sitting on the floor and holding up one end of a workbench. A friend is making brand-new fan shrouds for 4-cams, along with other parts:
Just found this: A REAL 550 (I think serial number 87) first start after sitting for 6 months. The guy starting it said it was rough because the gas was old (it smelled funny):