While cassettes are clearly easier to disassemble, it’s not that difficult to remove the cogs from a freewheel body. You’ll need at least one chain whip (two makes it much easier) for the task. If you don’t have one you should have one anyway and they’re not that expensive. Google the process. Once the rusty cogs are separated from the body, a 24-48 hour soak in EvapoRust should do the trick, especially if it’s just surface rust. Before reassembly treat them with some sort of rust preventing lube. Avoid heavy oil or grease as that will just attract dirt and/or other gritty grime. As mentioned, protect it from the elements as much as possible. Clean and reapply the rust preventive lube as needed.