You succeeded because these devices are almost always generic, they use the most widely available hardware
Very often, it is possible to flash device firmware that has both BT and FM receivers and more functions, even for components that are not the same. Of course all superfluous or inadequate ones will not work.
In addition to find the right firmware for the chipset, you should also be lucky to find the right resolution, density and type of screen (capacitive/resistive)
Be sure that the firmware is correct for GPS receiver on mainboard, that there are no redundant functions, and that the storage option is identical (most often 4GB especially if the device is cheap)

No, bootable WinCE does not exists
All unlockers/shells that are launched from the SD card work by bypassing the system launcher and starting only clean WinCE, which is the basis of every such device.

WinCE is part of the firmware but the boot section is separate.

You don't need copies of the "original" software or content from the device, it's not paid anyway. The Chinese steal it and distribute it on their devices (of course, they don't charge for it). It is cr*cked software, usually some older version with older maps

On this Forum you can find almost all the versions ever published in their best edition