My 3ft cracked the rear glass from the bottom corner at 45 deg angle to the top..... I merely smeared some (thin layer) silicon sealer across, left it for a day and refilled....... Can't help feeling uneasy about this one???
Buy a new tank. There's no point wearing a damaged helmet, just as there's little point to using a damaged tank.
Or at least get it dismantled and the back glass either repaired (doesn't sound likely with the description you gave) or replaced. Then refit and reseal the panels and you're set.