Paul, didn't you split the engine with the snapped belt that came out of my old breaker and find that the bores and piston crowns were badly scored as well as all the valves bent? Or does it tend to come out differently on BBY engines like that one, compared to the older AUA like the OP has?I’ve done a few of these and never had anything worse than bent valves. A set of valves and new stem seals. Head gasket and stretch bolt and it back together
Plus a water pump and cam belts/ tensioners etc
The parts will be around £300 maybe a little less but if you buy a second hand engine you would still need the cam belts water pump tensioners and then have the cost of an engine swap
So I think fixing your own engine would be as cheap as a replacement engine and you have an engine that you know the history off
Paul