buy an engine khoka - with no parchoon items on it - (thats karachi terms) in real terms it means
buy an engine with no manifolds and accessories.
Or buy a long block and rework the head,
You say that you are facing oil burning problems, heat soak, crank rattle and general sluggish acceleration - its called repairable. Now unless you are prepared to join the bandwagon of swap this swap that - go ahead.
You can look for the workshop repair manual for this car and then measure your engine to the specs. You will be amazed at how off our pakistani machinists are on tolerances. If you can somehow make them follow the book you would be set. AND its a simple, small tiny engine.