Why the mechanical pump failed or cracked? Did ur mechanic installed it without the plastic "Haddi" in between and tightened it too much?
Do following things:
Check if the mechanic has put a "jet" to reduce the pressure of fuel going into carburettor, some times they do it. if there is one do remove it.
Inspect the main pipe going from filler point and the small metal pipes connected with it going into the tank should be rust free and they dont have any holes.
inspect the rubber hsoe and pipes in between. they should be soft and not hard. they get hard due to the passage of time and doesnt let the suctions to work properly.
If there is a plastic filter in the line, remove it for the sake of checking.
While the tank is off, inspect the place where fuel guage is installed. it should be air tight.
The pipe going in the tank to suck petrol should reach the bottom easily.
Start the car and drive. check if the fuel pump is getting hot.
if yes, install a relay as it might be taking load and not getting the current.
do u have a return line?
for the sake of checking, u can also use a good quality fuel rubber line going into the tank and connect with the pump separately. when u check with it, it will also tell whether the original metal pipe is faulty or the fuel pump.
these idiot Chinese fuel pumps are highly unreliable and doesn't last long despite some of them contain checking warranty. they ditch you at the time when u dont expect it to be. I have faced similar issues in my car.