Due to the high legal requirements, and the extreme mechanical design and manufacturing requirements, I'd strongly suggest just buying a new semi-auto Thompson carbine.
These are available set up to look like either the early 1921/28 Thompson, or the WWII M1-A1 Thompson.
These are 100% legal to own, and cost far less than trying to build something from parts that ARE NOT compatible with a legal semi-auto design.
http://www.tommygun.com/
The problem with trying to build a legal semi-auto from an 80% receiver is, the receiver is 80% a full-auto and isn't going to be readily convertible to a true semi-auto.
After doing some heavy-duty mechanical design work and getting the ATF to sign off on it, you'd need to have a high-end professional-level machine shop with a full range of lathes and milling machines to actually build it.
In other words, you can't buy an 80% receiver and a surplus full-auto parts kit and do a few simple modifications to get it to both actually work AND satisfy the ATF's demands that the gun can't be "easily converted" to full-auto.
When George Numrich designed the semi-auto Thompson gun, he said that he'd spent $100,000 designing a gun that would satisfy the Feds.