I haven't really followed the cost of a Duetto body shell here in the US, but there a project car on US Ebay right now:
http://cgi.ebay.com/ebaymotors/ws/eB...&category=5356
It looks like it'll go for about $2000, looks like a good body with need of lots of mech work and, of course, paint.
I would think if you could get a good bare body over there for a $1500 to $2000 you'd be in good shape. If you can pick up the body with the rusted out floors really cheap, you can get replacement floor pans in the UK and have them welded in, assuming that the rest of the body compenents (and especially the inner rockers) are in good shape. What you don't want to do is pay a lot for a body that needs $2000 worth of restoration.
I don't have any experience with migrating mechanicals to a new body, but I don't see any problem, unless the new body is not straight (previous collision damage) and true. Of course, paying someone else to do it would be unbelievably expensive. Unless you have the time, tools, and a large garage, doing it yourself would be impossible.
In the end, it may be more worthwhile to find a really nice complete Duetto to replace yours. You may spend $10,000 or so, but might be able to offset it by parting out your car.
http://cgi.ebay.com/ebaymotors/ws/eB...&category=5356
Also, that way, you know what you're getting ahead of time. Did you have collision coverage? How much is your insurance going to pay you for the car? That's another cost offset.
I'm afraid there's really no inexpensive way to get the blue beauty back on the road where it belongs.