Hello: You can use any cleaners that are not combustable, though I wouldn’t spray in a way that could work its way into the heater. A rag with cleaner on it would be best for anything around the combustion chamber door. Beyond that there are cleaners using enzymes which are sold at Graingers and likely commercial cleaning supplies too. They are designed to do this particular job. A pet supply might have something to fill the bill also. If you have a wet/dry shop vac, you could get most of the moisture from cleaning away from the tank and pan with that. As long as the remaining water isn’t enough to puddle, it will dry quickly on a nice day.
Good luck 🙂
ps. Should you need to remove the pan, replace it with a plastic pan so corrosion will not be so much of an issue. Pans don’t cost much.