Just when I see some light at the end of the tunnel, now a catastrophe.
There is virtually no chance now can I bring it back to the way it was, because not only one 74HC00D chip is blown, the MCU (as the brain of the whole thing) is also gone. It won't work even with a new MCU chip because I will not have the code in it.
So I will have to go down the less satisfactory options of my 'multi-tiered' plan for the repair. What's common for all the 'tiers' is to have fun and learn things along the way. Other than that:
1) Ideally, bring it back to work the way it was (now virtually impossible); or
2) Repurpose the working parts of the machine if the first cannot be achieved.
Since all the motors and the mechanical parts are all good, it's possible to make my own control module (based on Arduino for example) to drive the stepper motors etc. to make the machine run.
Under this second option, there are a wide range of possibilities -- from making it barely run to turning it back into a fully functional sewing machine close, or even superior, to the original design (because I will be in full control of that can be done! Of course, anything on this side of the scale may remain only in imagination).
At the easiest end of this scale, it will be a plain straight stitch sewing machine, perhaps even without back-tack (which means it's practically useless).
Next to it, is to have the stich-length control, so also back-tack. This will be a practically useful sewing machine.
Even better, the sync related parts, both mechanical and electronic, may be brought back to action. One good thing is that those LEDs and optical sensors are still good. Due to its delicate design, I will still need to have the dead PCB in place to hold those LEDs and the plastic optical thingy in the right position relative to the main axle to make the synchroniser work. Anything starting from this point (i.e. requiring the synchroniser) will be challenging to me, even merely with zig-zag stitch ...