There are some things that I just replace. First thing I replace is the tires and I usually do that when I pick up the trailer Then I go to the top. I replace the vents. Unless the original is in fine working condition and water isn't coming in. So far I have not found one.
Starting at the top of the cost list would be the Fantastic Fan Vent. The one with the rain sensor and temperature sensor. It opens and closes according to the weather. Always closes automatically when it rains. That's a life saver right there. Expensive but very nice. If you go with the fan vent you will need to run some wires up there to power it. But you are opening up the top anyway so it should be no big deal.
I prefer vents with metal tops. Plastic ones just don't seem to last all very long.
God grant me the strength to restore the trailers I can, The courage to strip the parts off the ones I can't, And the wisdom to know the difference...