No start

My car had not been started for six months, prior to this it was working fine, but now its not starting. What could have gone wrong while it was parked?

I have replaced the battery, but still its not starting.

Mechanic said he ran diagnostics and found that it will fire spark on three cylinders before ecu shuts the coils down. He suspected the camshaft sensor as he was getting a crank speed signal.

He said he tested the sensor which revealed nothing saying its unique to the vehicle, suggested I get a replacement sensor directly from Mazda, but said the problem could also be related to a cooked ecu. Mentioned he opened up the ecu and inspected board and found no visible signs of damage.

Mention he was not sure if ecu needs to be re-encoded due to dead battery etc. And that a replacement ecu is expensive and a used ecu is hard to find for this car.

Any advice what to do? Cut losses, sell the car for parts or should I keep trying to fix. This is my first car.