Would it, in theory, be possible to complete upgrade the logic board?

You could always try these guys in China https://www.pcbway.com/ according to Northridgefix they offer to make custom boards..... long shot but it may cost 4 arms and 4 legs to get one made especially to your requirements.

Other boards are obviously designed for particular casings, so there are numerous things that 'won't be in the exact position' down to the holes where the screws would go to fit it in the casing. And then the technology to make stuff like the track pads and keyboards work as intended. Dosdude did something quite unique on a 17" before https://www.reddit.com/r/hardware/commen...

Sorry this series uses a custom logic board and not upgradable beyond the Core2Duo. The next in the 17” series also is not upgradeable beyond Sandy Bridge i7.

Keep in mind there is limits within the OSs these systems use which then limit you on what applications you can use or even get.

I too shed a tear when my two systems finally failed as I was expecting more than they could handle and couldn’t run any newer Applications I needed, I now use newer 15” MacBook Pro’s.

Yes, unfortunately, you cannot really 'upgrade' the motherboard as there are limits. The motherboard on its own costs pretty much the whole computer, so you may as well treat yourself and buy a new Mac.