It would be cast iron.
Welding/Brazing might make an issue with "warpage."
You might "flatten" it after the fact with sandpaper glued to a tablesaw table.
JB weld might be a better fix but you have to "grind" the surface to make a place for the epoxy to "grab hold."
Actually I've seen a perfectly good cover on Ebay for $8. I bought it because I saw it was actually a "Model B" cover which I wanted for use with a Gordon-Smith Air Compressor conversion - which uses a Model B centrifugal distributor to match the "load-no load" condition of a compressor engine. (cover/timing pin and Model B distributor have to be used as a pair to allow "normal" engine timing.)
Ebay: $14.88 plus ship best price currently.
To tell the differences between covers, see Vince Falter's page on the covers. The page doesn't seem available at this moment - perhaps Vince is working on the site?
www.fordgarage.com
Joe K