Old Bike, New Parts
 

I am considering taking my son's bike, equipped with Sora, stripping it and putting the modern components on an Austro-Daimler Steel frame as a commuter. I'm guessing that I'll run into problems like FD clamp fit/braze-on issues and bottom bracket issues.

Questions I have: How do I know what size FD?
Will the bottom bracket swap out or will I need a new one? How will I know?

Front derailleur clamps come in 1-1/8", (28.6 mm), 1-1/4" (31.8 mm) and 1-3/8" (34.9 mm) diameter so a caliper on the seat tube should be able to tell what you have. I assume the Austro-Daimler requires a 1-1/8" clamp and the newer bike has what ever it measures.

The new bike most likely has English bottom bracket threading and the cups will be engraved something like "BS 1.37x24" . Look at the A-D's current bb and see what is engraved there. If it's English you are ok but the spindle length from the new bike may not give the correct chain line.

You are also likely to have rear dropout spacing issues. The A-D, depending on it's age, could be 120 mm (5-speed), 126 mm (6/7-speed) or 130 mm(8-speed and above). The new bike will almost certainly be 130 mm.