I have a fabricated lawnmower that has the top deck corroded through,
and I decided to replace it with a sheet of stainless so I'll not have
to do the job again. I have one problem. The original deck top has a
rounded front of 12.5" radius, and all around that radius, as well as
down both sides, there is a rounded down edge of 1/2" radius to make
it fit nicely on the 1" dubular steel frame of the deck.

How can I form this radiused "lip"? I'm not sure what guage it is, or
even what kind of stainless - it was a "surplus" stainless steel
countertop. Is there a way I can anneal this to make it easier to
work?

I'm thinking of making a hardwood block with the right radii in the
end grain and going at the job with a 2 lb hammer. The block will be
pivotted from the center of the sheet and swung around as I hammer my
way around the semi-circle - the straight sections will be easier -
but I'm not expecting it to be a simple job.