G'day one-eye-Tim and welcome to the forum.
Is your donor engine in good running condition?
If so I'd be tempted just to use it and source some twin carbs, hotter camshaft or maybe bolt on a turbo at a later date. These cars are more about the diy build and the driving experience than pure performance. The finished article is quite light so a little power goes quite a long way.
The cars also have limited protection from the elements so even when you're going sub 100kph you'll feel like Burt Munro going flat out on the Bonneville salt flats. Fantastic fun even when granny is sitting behind you and flashing to get past.
After all, the original company demonstrator used a 948 and still has it as far as I'm aware -
Good luck with whatever direction you choose to go with your Miglia, looking forward to seeing it progress.
Oh, and most of us use Photobucket to upload pictures.
Try
www.photobucket.com and follow the instructions.