If you decide to repair it the absolute minimum is going to be the head gasket being replaced. This is a fairly labour intensive job on a stock car so would be costly on its own. You may need to add in a head skim, you might as well do the valve stem seals while at it and you would need to find the root cause of the overheating (radiator/thermostat related). Not sure what this will cost but it will have 3 zeros after it regardless.
Replacing the engine is straight forward enough for most people, labour would most likely be less than with a head gasket change but you would be running a sort of unknown engine. The engine would cost you about £1000 also before any fitting etc. Given this scenario you would still need to find the root cause of the overheating so that makes no difference to the cost.
If it was me I would do a compression test to see what readings you get. I'm not sure how much difference a blown HG would make to those results though. If the compression test results were favourable I would fit a new gasket, refresh the head, change the thermostat and flush out the cooling system. Job total would probably be around £1000-£1500.