Might the System 3 board be fixable?

I'm tempted by the motors as I've used the ones from my milling machine on the router. I can make you a breakout board, but that's not really worth two motors ...

What motors were you running on the 2.5A drivers? If it was the 3Nm ones you would be better off running them on 4.2A, in bipolar parallel ... with two PM752 drivers (or equivalent, there's cheaper ones on eBay) added to the power supply I did for you.