White smoke coming out of tailpipe

I have 2003 lexus is200 2.0 litre automatic. White smoke comes out of it. I bought the car 2 weeks ago and it was okay until I filled up half of the tank as it was almost empty when bought it. Took it to my mechanic he replaced gas kit n cylinder head but its still doing it. He did check the petrol and there is no water in it. He is saying last option is to replace engine. Car doesnt get ocver heat, drives perfect but its white smoke. Please advise. Thanks.