New axle assemblies aren't terrible. You can find a brake axle for around $4-$500 each. I think the trick might come in mounting the new axle as the ones you have pictured are essentially house trailer axles. Most of them I have seen have a fixed bracket on one side with a pivot in the center between the two axles. I never understood how they rate that type of axle.
Technically you would identify the capacity of the trailer by having it weighed and then reducing the load rating by the weight of the trailer. If you have two 7000 lb axles and the trailer weighs 4000, you can gross 10K on the trailer.
Some manufacturers cheat and use tires rated at 3750 each rating the trailer at 15,000 lb gvwr.
On Amazon:
https://www.amazon.com/000-Electric-Brake-Trailer-Axle/dp/B06XV5CF2C/ref=sr_1_11?ie=UTF8&qid=1514655415&sr=8-11&keywords=7000+lb+trailer+axle
On Ebay:
https://www.ebay.com/itm/2-7k-Electric-Brake-Drum-7000-lb-Trailer-Axle-RV-Dump-Cargo-Flatbed/302027527171?hash=item46523e5403:g:KwcAAOSwr~dZknW R&vxp=mtr