Been there, done that!
A piece (or two) of 1/8"x1/2"x2" aluminum angle will act as stops and supports for the broken bolt downs. The weight of the fuel will keep the rest in place.
Relocate the vents as high as possible (well above the rubrail) to reduce fuel from escaping when in rough seas and trailering when full.
From what I remember, the vent is 5/8". The new fuel/vent hose is EXTRA thick, I had to use (either brass or bronze) 90 degree fittings, think it was named "Quest". Same fittings for poly pipe in houses.
Like Reel and MJ said by moving the gas vent to above the rubrail.
In the original vent location, instead of filling the hole I just put a new one in. Hell $6 each side is easier and cheaper than fixing the holes. ;)