I've only found two mobos that use this chip, it's rare. I need to buy it on aliexpress. The chip on my mobo is marked:
PC87309-IBW/VLJ, with MEGATRENDS writing, suggesting an American Megatrends/AMIBios
Chip from aliexpress is marked:
PC87309-ICK/VLJ, with PHX writing, suggesting a Phoenix Award bios
What do the prefixes ICK and IBW mean? I hoped it was just a revision of the same chip. Datasheet only refers to PC87309 marking. There is no mention of the ICK/IBW prefixes inside the document, which would suggest that I could try using a chip from aliexpress. As an attachment to the post, I am also adding a file explaining how to read the Nacional Semiconductor chip markings. Unfortunately, I don't see an explanation of what ICK/IBW means. But maybe someone else will be able to.

However, I am worried about the markings indicating a specific type of BIOS (MEGATRENDS/PHX). Could someone check in the datasheet whether the chip has its own memory, or in other words, is it possible that this chip is programmable?

VLJ seems to mean QFP100 package.

There is an internal mask ROM in these chips but it is the keyboard controller firmware, which in theory is compatible regardless of who made it unless there's some custom command that only one type supports and the BIOS of the motherboard happens to rely on it but I think that is unlikely.

But seems you won't have any options and will have to bite the bullet and get one chip and do a replacement.

Chip has been replaced by ICK version from aliexpress, motherboard is 100% functional now 😁

Excellent ~

Great ! good skills and glad the china part worked.