I have 1970 Fiat 500f and suddenly bonnet support was detached from the panel. Seems like it was not attached correctly as it has rust on the surface, should I weld this spot? Any advice appriciated. Thank you very much.

Originally that was spot welded on each tab. A good method would be to drill a 7mm hole through the panel above each tab on the bracket and mig-weld through whilst all clamped up. You could probably get away with pop-rivets or small bolts (or maybe PU sealant for an invisible repair) but purists might disagree.

Thank you very much for your advice! I will try that method!

If you know somebody with a synergetic welder, you could drill it and mig braze it. Lower temperature would distort less, cause less damage to surrounding coat of paint and the weld will not rust in days. There's also the advantage of capillary action of the weld will give greater surface area of the weld so stronger.