I do not know the tubing gauge but I can measure a piece of left over frame tubing for you. I have old Warriors and newer Warriors and have not noticed any difference but I haven't measured them either...
14 gauge sheet metal would be sufficient for the neck braces. You can go thicker if you want but I wouldn't use less than 14 gauge. 1 piece on each side. 11 gauge is 1/8'' (.125) and 14 gauge is 5/64'' (.0781).
The engine plate can be reinforced underneath with additional welding and a few struts. Or you can overlay a piece of 11 gauge on the top. Gusseting the lower rails to the rear wheel hoop is a good idea as well.