i cut a bit section out and welded in a patch but i made it look as it was not just the number cut out, a mint m4 shell is def the way to go cause you will struggle to find a decent m3 one, basicly if you restore your s1 you will be replacing the same panels anyway so theres no difference,
replace the-- scuttle panel, outer wings, front panel, bonnet, back panel and the boot, there you go, m3 shell, rear quarters are the same, doors are the same,

if you wanted you could grind out your old number, get it wet blasted, tidy it up and weld it in your new shell,