Door panel is a Mitsu only item. Buy it at a dealer or a pick n pull/salvage yard. Car-part.com is a good place to start for used parts.
roof rack: any gutter mount system should work. I'm currently running a 7' long Garvin Wilderness Trails Rack on mine (after i removed the OE rack). Before that, I ran a bvg rack with Yakima load bars.
Bumpers: ARB, TJM, or ECB make Montero bumpers but new they aren't cheap. If you can find a used bumper for a similar width rig for around $250-400, then have a welder fab up a bumper mount mod, it will be cheaper than a new ARB unit.
In my case, I bought a new smittybilt front and rear bumper then modified things to get them to fit without modding the bumper.
Air Lockers: for the rear, there's the venerable Mitsu 9.5" rear diff & oem air locker setup. For the front, ARB makes an air locker for ~$1000 and IIRC, you can also buy an Aussie Locka for the front for much less but its not selectable.
as for what swaps... The motor is Mitsu, the drivetrain is mostly Mitsu with some Aisin thrown in occasionally. Even the R&P's are Mitsu products. Thats because as a company, Mitsubishi is a literal super conglomerate with an umbrella of companies in many, many industries, not just cars/trucks, but electronics, heavy industries, etc.