Are you going to be towing on the road, or just using for launching at a boat yard? If the latter, possibly not worth the effort/cost to fit brakes, as they aren't much use for launching/recovering, and just rust away again!
My trailer similarly came with all the brakes stripped out, and I reinstated for road use. You could buy complete Alko backplates with all the components for ~£80 at the time, so I just bought 4 of those and swapped the components onto my backplates, along witj new bearings, which at that time was far cheaper than 2 new 1600kg axles. But if you need hubs/drums and other bits as well, the cost might be much closer to completely new axles.
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