Reinforce the rear bumper either by welding additional reinforcement plates on between the frame and the square bumper or by using one of the commercially available reinforcement kits:
https://www.amazon.com/dp/B084ZTS882
Then, bolt a 2-inch heavy duty receiver onto the bumper:
https://www.amazon.com/gp/product/B0007M1ZGE
Then the sky's the limit for bike racks. A bike rack can get pretty bouncy back there behind the axle on a spring-only trailer, so bias toward stout carriers that secure the bike by the frame.
Searching the forums for "bike rack" will also give you more info that you'll know what to do with.