@ebinbenis As this and other answers have mentioned, it's the stuff like gaskets, seals, hoses, belts, etc. that will be tough to acquire. After 90 years, it will all be unusable, generally including the spare parts that had already been made.

Without a modern supply chain, making rubber parts won't be easy, especially considering what this answer discusses about the number of people that will need to be focused on things like making more people and basic life necessities. The modern supply chain requires a huge number of people specializing in very specific things.

The airframe itself on a B-25 would very likely still be mostly airworthy and any spare parts made for it will likely be fine. Most of the aircraft I've personally flown were built in the 1960s or 70s and remain perfectly airworthy today.

Many WWII-era planes are also still flying around the world. The engines, however, are another story entirely. Those must be constantly maintained and non-metal parts completely replaced, which will be very hard to do. Of course, the tires will also need to be replaced, which won't be easy. Rubber doesn't last long.

As an interesting point on that note specifically regarding WWII-era tech... lack of rubber was actually one of the biggest problems that the Japanese faced, along with lack of fuel. Their navy was very nearly useless by the end of the war just due to lack of oil and rubber (supply cut off by the allied navies during the war.)

And that was a country of tens of millions of people with a modern supply chain. Their supply chain largely collapsed in a matter of a year or two without oil and rubber.

I was thinking of fabricating some sort of temperate stand in for rubber, because, as another comment added, the addition of a fantasy world/elements means that you can be creative. Whilst I feel that replacing the engines with a magical standin would be lazzy and overly plot-convenient, the addition of some sort of polymer-producing plant would hardly be too inconsistent.

Also, they would not be restoring the entirety of the systems. Some things, like the hydraulic systems powering the, and Norden Bombsight, would be far too complicated to restore to working order at their current power level. They mightn't even be able to retract the landing gear. Nonetheless, even at a reduced capability, it will fly. They're more or less using it as a low-level eye-guided bomber and strafing gunship, hardly an impractical use when it has 6 forward mounted .50 cals.

@AlexP The civilizations you mention do not exist as civilizagtions anymore. That is the difference.

@reirab "That was a country of tens of millions...' Ah, yes, the Catch 22. Tens of millions of people DO need a LOT of resources. A few hundred thousand woul dsurvive very nicely for decades, if not centuries, on the oil we already have in storage. Rubber is not hard to vulcanize, and large quantities are not needed.

@ebinbenis Polymer producing plants already exist. Latex comes from plants.

I also wouldn't try to do it in anything smaller than Great Britain, with access to the ocean for materials that aren't available at home. To get an idea of what sort of materials you need, this site has a list of critical strategic materials during WWII. You could probably scavenge the metals, but coal, oil, sulfur, phosphates, potash, and rubber would all need to be acquired fresh.