Stock ROM can be restored by using either PC Companion or SEUS. If your recovery still works (assuming you installed it after rooting), you can install a custom ROM immediately without having to revert to stock.
Manufacturing date is only of relevance to you if you're planning to unlock your bootloader and flash a custom kernel, which in your case is not recommended. Check the link Gogeta gave to you to see why this is the case.
At this point, the worst thing that could happen is that you soft-brick your phone by flashing a ROM that requires a custom kernel to run. And even then, you can easily revert back with PCC or SEUS.
Search for a ROM that requires a stock kernel, and follow the instructions. Perfectly safe to do.
Device: Sony Ericsson Xperia X8
ROM: FroyoPro v2.5
Kernel: Stock (11w41)
Clock: 245/729 MHz, Smartass