Brother I think you need to sell your car and get a carburetor engined old car which your mechanic can experiment on.
I fail to understand why you refuse to bring your engine to factory specs. There are hundreds and thousande of M13 in various cars running flawlessly around the globe.
Remove timing chain cover and check for leaks near water body, then underneath the car and then radiator and all pipes. All of this needs to be done when the car is hot or steaming.
INSTALL THE THERMOSTAT.
Check for a few days if there are any other leaks and then pour coolant.
Itherwise, sell it as is and buy a Mehran or Alto.
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