A full rebuild is around the £4K mark, if you only need the top half of an engine it might be cheaper.
Here's a couple of places / people that might be able to help you source an engine. They can be found from £2000 to £4000.
@Samwise has one for sale - see attached image.
Abbey Motorsport,
Fensport,
Tuning Developments,
Rogue Motorsport
Sam Nadeem - https://www.facebook.com/Saam27
Matt Baker - https://www.facebook.com/MBDevelopments1