Being the car that it is you are better off fixing your engine or just getting rid of the car, I do not recommend a used engine replacement in an astra / vectra at all as most of these suffered the same fate as yours has.
It would also probably end up costing you the same to do a head gasket, timing belt etc as it would to get an engine supplied and fitted.
I did come around around the other day, the corolla was out the front but the shutters were closed.