agu with a ko3s would be a good option as the agu engine is fully forged and the ko3s turbos are now inexpensive to buy and bolt straight on, and down to the engine problem id say if the belt is still intact then id of thought the coolant has probably dumped out into the cylinders causing it to hydro lock, if you have chance before the garage get it whip the plugs out and see if it then turns over chucking all the water out, if so the head will be warped/cracked so a new engine would probably work out best