Engine help

[savageman25]

My mates got a mk2 ford granada 2.3 v6, he bought it off one of his mates for banger racing as his mate said that the head gasket had blown and we were going to put a 2.8v6 lump in, thing is though, is that we gave it an oil change and had a go at getting going just incase the original owner was wrong about the head gasket, after a bit of persuasion the engine fired into life, ticking over lovely and revving real nice, no creaming of the oil, no horrible smoke from the exhaust, we had it running for a good 30mins-hour just tinkering with bits a pieces making it run spot on, so two days later my mate went to start it again, nearly as soon as the car started the engine, it was backfiring,miss firieng,eratic idle speeds, and lots and lots of white smoke from the exhaust, im just wondering what could have caused an engine tht was running sweet as a nut to be a dud two days later.
I no alot of you would be saying whats the point seeing as its a banger, its just that the 2.3l v6 ford engines are really hard to come by these days and are in my opinion very good engines. (apart from fuel costs lol)