At the back of the above mentioned book there are details of clubs and specialists in various countries - but bear in mind that this book was first published in 1993...... (I think there was a later edition also)
Middle Barton Garage is mentioned.
Abarth Club of GB - Tony Castle-Miller (who iirc is owner? of MBG)
DTR European Sports Cars - used to be big in 124 Spiders but I think have now progressed to Ferrari's etc.
Guy Croft Tuning - expert on Fiat tuning, published his own book on tuning.
I remember a Motoring Journalist by the name (iirc) Ross Alkureshi. He ran a Fiat 124 Spider Abarth, possibly a Stradale, while writing for either Classic Cars or Classic & Sportscar magazines. He did a series of articles documenting his experiences of running this model incl having some front end accident damage repaired. He subsequently sold the car on. This was a few years ago but you might be able to locate back-issues of the relevant magazines if you wish to read them.
My recollection was that, at that time, while standard 124 Spiders sold for £6-£12K. depending on condition, the 124 Spider Abarth seem to be offered for sale at maybe £25-£30K.
(not sure if this was the Stradale version or the much rarer Rallye version).
As the Stradale only has 128 bhp (from a blue-printed 1756cc engine) and the car (because of the different suspension) was described as crude for road- use, I think I'd modify a standard 124 Spider (for which parts supply is excellent) and save the difference.
If you check out Guy Croft's book, you'll see just what can be done to 124's (He has also raced them successfully).
AL.