ORIGINAL: bomccarthy
At least with the Piper, you have a toy which you can use fairly often. Parts should be easy to come by and the later versions of the engine are still made, unlike with the others. If you own anything with a Merlin, you'll have to stand in line for parts with the racers, who blow a few up every year.
There are enough Merlins still flying to find spares. There was a company in Chino that specialized in spare parts for Mustangs. They carried spares of all the usual maintenance items and could make anything else. The owner had a Mustang and gave me a ride once.
The more powerful the warbird, the more expensive the parts will be though. When I rode the Collins Foundation B-24 I was talking with one of their ground crew and he said the cost of fuel was one of the smaller expense items. Spare parts were what ate up most of the budget. He said the main landing gear tires need to be replaced a couple of times a year and cost $1200 a tire. Fortunately the B-17 and B-24 shared the same tire size, but they aren't regularly made anymore. Continental will do a special production run for them and they pool with other B-17 operators, but they are still very expensive.
There are specialty companies making spare parts and restoring engines, but anything from a boutique business is going to cost a premium.
The trainers are popular in large part because they are a lot cheaper to keep flying. There are a lot of them and they are fairly cheap. Once you start getting up into the 1000+ HP arena, the prices skyrocket both from demand and extra expenses keeping higher performance planes going.
Bill