its a hard one to call really as i did the work myself.
it cost me around £520 for all the bits and some of the real engineering work to be done, ie 4 new valves on the culprit cylinder, crank reground, head skim, new con rod, refaced piston and new bearing set. i did get all new engine bits too like new belts, new tensioners, full engine gasket set and new cylinder head bolts.
i would think you are looking at an easy £3k if you pay someone to do all this work, its not a job to take on lightly but if you know what your doing.......
said though you can pick up decent Twin Spark engines from alfa specialists or even from an alfa breakers. i wouldnt expect to pay more than a few hundred quid for a complete engine.
one last query, how high is the oil level in your engine? i only ask because mine was over-full by about a litre and it caused all that damage, if the oil level is over max i would have a word with where you bought it from. if there it too much oil in the engine it will put a hell of a lot of pressure and strain on the crank, resulting in the crank bearings disintegrating. let us know how you get on anyway and if you need any help give us a shout fella