Yep, we already do. This is cycle accurate

https://github.com/MiSTer-devel/Gameboy_MiSTer

Edit: We can also do GBA and a lot of other cores, all the way up to Playstation 1.

Could you explain a bit how your project is different from the one in the article? Why did they implement the SM83 in HDL when you already have such an implementation?

Also, I don't see any specific FPGA's mentioned, what sort of constraints do these projects have? Does it run on a 3$ one, or do you need something bigger /fancier?

Edit: oh sorry didn't realize MIST is the device, it's an Altera Cyclone EP3C25

The one in the article is based directly on the die scans, while the MiSTer one is a completely custom implementation.

How is the custom implementation confirmed to be "cycle accurate"?

Not just Game Boy but much much more:

https://www.analogue.co/pocket

I still have that one on my wishlist, there's so much good stuff on the original Gameboy systems that I never could play - our birthday budgets didn't stretch nearly far enough.

There are already products doing just that, the Analogue Pocket comes to mind (https://www.analogue.co/pocket)

GateBoy will eventually do that for the whole reverse-engineered die.