Sounds kinda simplistic - but
What, and where the "what" was, made it rust?
Eliminate the cause and the rust stays away. - At least until next winter.
Uncoated metal?
Sand and paint it.
Leaves from trees (good source, huh)?
Cut down the tree, or park someplace else.
Plugged drainage holes?
Don't think there are any in the cowl, but something is happening to trap moisture in there.
Find that, and with a little paint, you're good to go.