1999 Mazda MX5 repair or replace

The water pump gasket was initially replaced on my Mazda MX5 by a qualified mechanic who I have used previously. Then the car wouldn't start, there was pressure in the engine. He said that he didn't have the tools required to replace the head gaskett which he thought it might be. The car was towed to a local garage who said they had the required tools and would replace the head gasket at a cost of  500
They replaced the head gasget this being the now diagnosed fault.
It has been skimmed and pressure tested.
The car wouldn't start.
They then replaced the Crank pulley and Cam sensor, said they hadn't noticed that the crank pulley was damaged.
The car wouldn't start.
They called in a man with a laptop who said the only other thing it could be was the coil pack.
They then replaced the coil pack.
My car will not start. I have a bill for  789 and will have to tow my car back home.
Their advice is to get a new engine.
The car had a water leak, under pressure only. It drove like a dream but I obviously didn't want to cause any damage so took it in to be repaired.
I don't know what to do next. Why should I need a new engine when all this work has been done.
Many Thanks, Lynne.