SSD,
With enough time, money and will-power you can make or source almost every part. One of the notable exceptions, for some reason, is said to be the undercarriage selector. I imagine the process will in conceptual terms be something like:
1: Jack up undercarriage selector
2: Replace aeroplane
3: Jack down undercariage selector
Takes a few years and a seven-figure sum, of course.
Whether you end up with a Spitfire, a Spitfire composite, or a replica, is a kettle of fish best left to the philosophical or religious.
Back to the original question. A senior instructor involved in converting people to warbirds at Duxford recently confirmed to me what others have said: the best preparatory experience you are likely to build at reasonable cost is flying a Chippie from the back seat with a hulking great mate in the front. On a more pragmatic note, he also reminded me that the ability to navigate safely and accurately around the countryside with no more than a compass, map and stopwatch, with no GPS or navaids, at anything between 180 and 240 knots, with limited fuel endurance and not a lot of view ahead, is also highly relevant.