what is wrong with my transit?

Engine started rattling on start up but van still drove ok, after about 20 mins van lost power and cut out, took oil cap off and smoke poured out but there was not a shortage of oil. Later in the day van started gain , but again was rattling very loudly. turned engine off and left it. some people i have spoke to reckon a new engine, others think it is pre-tensioners/timing chain problem