Toyota 15cc diesel do not start after ss

hi , i have a Toyota 15cc diesel engine (automatic gearbox) overheted and needed head gasket replaced and a new head fitted car went perfect for a few weeks but burning a little bit of oil. then suddenly the car stopped starting in the morning or after the car was not in use for a few hours… the only way to start is to keep turning over and over until it start and when it start a big cloud of white smoke exit from the exhaust for about 3 mins working , I have also changed the injectors and the heaters but the problem is still there . can you psl help me solve this problem ???