The method described in this thread works for me, though some people appear to be having trouble with it:
As noted in that thread, you can also use external tools like FSUIPC to create flt files rather than the wonky in-game trick, but I haven’t tried that myself.
It looks like the same save system 2020 used, though it includes some new 2024 data such as the wear & tear system. Probably not all planes play nicely with it (2020 had plenty of issues with this save system on complex aircraft).
.flt files are human-readable text files and the wear & tear information can be viewed and edited if desired. You can also view and change the wear & tear data by turning on Developer Mode and using the debug menu.
You’ll need to use at least one of the above two methods to edit the data if you want to simulate individual-part repair, since the career mode maintenance interface isn’t available in free flight. (Of course you can simulate overhauling the whole plane back to brand new just by starting fresh instead of loading the flt file.)
Only planes made for FS2024 support the FS2024 wear & tear system.
Something else to note is that some (most?) planes don’t let you put covers/chocks back on once they’re off, but you can use the Developer Mode menu to do that as well.
Also, I posted this in that thread as well, but make sure your pilot avatar is inside the plane when saving the flt file, otherwise the file will be missing most of the information it should have, including wear & tear.