Hi, I guess welding up the 4 stud holes then redrilling in 5 stud configuration is an option if it doesn't affect strength, balance and there is enough diameter in the hub face. Do take a look at the back of the hub face too - most modern ones have thick and thin parts to get lowest possible weight. You wouldn't want a stud in a part that was thin!
Swapping the whole hub is an option and relatively easy when the hub doesn't include a splined hole for the drive shafts. Here is a link to photos and description of how someone put a bigger 5 stud hub from a Mazda RX7 in place of a smaller 4 stud hub on a Mazda MX5 (Miata). This was on the front so no drive shafts involved.
http://www.cardomain.com/ride/2308228/4
As far as I know the Alfa 166 has the same PCD and centre bore as a Dino wheel. (Same company after all!) Dino wheels were the same on Fiat Dino and "Ferrari" Dino 246 GTs. Of course the Alfa 166 is a front wheel drive car so maybe it's rear hub could go on the front of a Deon with a little modification. I guess a front one with drive shafts could be adapted to the rear.
Failing this you might have more luck by contacting Gerry Hawkridge of Hawk cars. He makes Stratos replicas (which originally had the Lancia Beta bits that the Deon used) He makes 5 stud conversion kits - in 5x98 and , I think, 5 x 108. He also used to make Dino replica parts.
enquiries@hawkcars.co.uk (5 stud kit is about GBP527+VAT)
Cheers
Ross