You should try measuring the components in continuity mode to find shorts, you could start with the capacitors marked with C, then measure resistances of components marked with R, the ones marked with Q might be transistors which can fail.

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My guess is maybe the fan had an inductive spike that damaged the boost circuit that steps up the cell voltage to 5V. A short on the USB ports should not kill it, that said you can never be sure with lower quality chargers, but they usually have over current protection

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The issue with circuits like these is that the issue is likely in the boost IC which may be hard to replace, if it is an issue with some passive component like a failed capacitor or a blown resistor then that would be easier to fix, but then there is the issue of the tiny SMD components which require some skill to replace

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The diy method depends on what kind of polish you want, you can find barebone circuit boards which have 2 usb ports, one charge input and battery terminals, you can find similar with an enclosure. I would recommend buying these on places like Aliexpress or Banggood as the prices will be better

You can generically search for DIY powerbank or DIY powerbank enclosure,

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Just the circuit board: https://www.amazon.com/Alinan-Charging-Overcharge-Overdischarge-Protection/dp/B09XQ8PQZ8/ref=sr_1_10_sspa

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make sure to look at the voltage that is supported, for a single cell pack it will be 3.7V - 4.2V like the one I linked, many are for 18650 cells which are common cylindrical cells, the circuit board doesn't care if it is cylindrical but may are for multi cell packs so would be with higher voltages

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Your pack looks like a single cell but you can measure it to confirm, it should be around 4V for 1 cell. Each cell might have a voltage between 3V - 4.2V so you will measure multiples of that based on the number of cells in the battery pack