The ones you mean are Ricambi. Have met them few times and bought spares from them as they are local to us. Very nice people. Don`t know how much they are selling the engines for, but I`m sure they will give you a quote. Also the price probably depend`s on, if you want all the fancy stuff("Abarth" sump, rocker cover, exhaust...)
There is also Ricambio which is a different company. They are selling they recon engines on eBay for £775,- with unleaded cylinder head, but this is just for the engine.
I think this is a lot of money to spend. Maybe you should try to price up all the parts needed for an engine rebuild and find a good old school mechanic who could do the work, or possibly do it yourself. I think this is a cheaper alternative, but not sure. It may cost the same at the end.
Good luck...