depends exactly what is wrong, but costs will be high if there is a vale issue or other cylinder issue requiring engine replacement.
£2000 or more isn't out of the realm of possibility.
likely yes, but not 100% necessarily. this is why proper hands on tests by a garage that knows this engine well is required.
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if the amount of labour required to fix the issue is more than the costs of just swapping in a used engine, then you will probably want to swap in an engine.
it probably would not make more sense to trade the car as a car with a blown engine is not worth much on trade value, so you would be paying almost the entire cost of a replacement car vs. just the cost of a replacement engine. This is something you have to decide based on costs to repair vs. replace, and your own financial situation.