Well done, you have one of the easiest cars for an engine conversion - seperate chassis and easily removable panels.
You don't state the year of your Spitfire but all of them come with twin carb versions of the triumph 4 cyl engine - 997cc to 1500. A relatively easy initial swap could be a single carb setup from a related Triumph with the same engine - For example if its the 997, 1200 or 1300 you could get the single carb setup from a Herald (same car underneath) and save some there.
For a federalised engine the 3 cyl Metro (or even the 4cyl one TBH) would be best for FE, the Miata plant will give more go but its not great for FE. As an alternative how about the engine from one of these ?
The engine is a 657cc 3 cyl turbo, and would fit perfectly and it comes with an inline gearbox attached which can be bolted straight in. They are certificated for Japan which has similar emissions regs to the US.
Think carefully before going too much bigger (TDI etc.) because of the weight - the spitfire has a swing axle rear end which is bad enough at the best of times, but the front suspension is much better.
Obviously consider upgrading brakes etc. at the same time.
Good luck - lets have some pics