the overheating part is because you were probably running it on plain water which blocks the radiator lines, secondly this engine gets airlocked pretty quickly and shows itself as full when infact there are air bubbles trapped in it, proper bleeding procedure for it takes at least half hour, and never use regular green coolant in it, the Nissan green coolant is exactly the same the new toyota red which is a HOAT coolant with no silicates.
Thirdly you mention that the camshaft was replaced, did you replace the respective shims too as the cam and shim wear a pattern together, they rotate while being pressed. If not you might have flat cam now, which is terrible.
lastly direct fan, tstat removed, engine hack job etc, its best that you look for a very decent workshop or buy some diagnostic tools - like an oil pressure gauge and compression tester, check both and then proceed.
To me it seems that you will see some very interesting results, interesting can range from bad valve seals, glazed cylinders or even a plugged breather causing toxic sludge to form and kill your engine.
Nissan engines are very good, and have a great service life, but for some reason they dont in Pakistan.