@JustinThymetheSecond The oil and rubber mostly weren't going to the people, but rather to the war effort. They still ran out. Granted, this question isn't fighting against a couple dozen Essex-class carriers and a couple dozen more battleships along with hundreds of other vessels and thousands of aircraft, but maintaining the supply chain to keep fresh oil, rubber, etc. is going to be nearly impossible for a society of a few thousand people total.

How much you start with is only marginally relevant when it's already 90 years old. Much of it will already be useless, regardless of how much you have.

Piston engines are surprisingly complicated pieces of technology. The ones built during the WWII era are perhaps the most complicated piston engines ever designed. Almost immediately after the war, design of new large piston engines for aircraft completely stopped and all even marginally heavy aircraft switched to turbine engines, which are far more reliable, but require high-precision machining.

The B-25 uses a pair of 1,600 hp Wright Cyclone 14s. For comparison, most piston-engine aircraft these days use a single engine at somewhere around a tenth of that power (that is, about a twentieth of the overall power of the Mitchell.) And even those require frequent maintenance and overhauls to remain airworthy.

Also, while the level of precision manufacturing of modern jet engines isn't required for these, it's still some pretty impressive machining for its time. Basically all of the fighter and bomber engines of that era had superchargers or turbochargers. It's needed in order to get enough air into those things, especially at altitude.

How many people would you imagine are needed to maintain the industry of keeping one (infrequently flown) B-25 serviceable?

Keep in mind that when I was referring to the 6,00 number, it was in regards to the main settlement, which is built around the hangar/airfield, and sort of serves as their industrial center/military base/capital.

They have a network of farms and villages which they receive food from(With varying degrees of coercion), and thus need to involve very few of their group in food production, though a decent amount of their number is involved in logistical and industrial efforts separate to making planes, leaving maybe 1,500-1250 to focus on aviation-based matters. Is that a realistic number?

They also get the lion's share of trained technicians and engineers, as well as tech and resources, due to the leader being decidedly invested in air power.

@ebinbenis The Canadian Warplane Heritage Museulm keeps their Mitchell flying with only a few dozen volunteers. They have a complete machine shop where their machinists can produce any necessary parts.