Can I use PCI-Express card slot (/54) to override internal graphics card on laptop?

It’s worth noting that the GPU on a laptop is usually connected (either internally on the GPU or an external chip) to a LVDS which is what sends the signal to the LCDs controller in order to render images on the screen, it may be possible to connect something up inside as LVDS controllers can be picked up easily online, however you might have trouble finding space inside a laptop in order to fit that in, as well as crazy connectors OEMs will throw in there.

The other issue you may have is BIOS support, not all laptop motherboards will support hooking up a external GPU to them, especially if it’s internal, it may even be possible that the BIOS has a lock on it preventing most hardware being added to them. (my current laptop which I’m using has a internal mini PCI port, which has a BIOS whitelist which would prevent me from plugging in a wireless card from another vender).

So, it is possible, however you probably are just better of getting it fixed, or if you really want to deal with the problem for good, see if you can find a suitable laptop online, if the same system board has died twice on you in a few years, it’s usually a sign it’s probably only going to happen again.