Unless you boil this pan past the point of dry, the inner surface will not exceed 212F. Just about any spray paint will work just fine, prevent further rust, and not outgas after initial drying/curing. The typical acrylic or enamel spray paint is considered hygienic grade in the food industry because after drying/curing they can be wiped clean with typical ammonia or bleach disinfectants without breaking down.
True "food-safe paints" are available and are intended for long-term food storage directly against their surface. They exhibit extremely low bisphenol-A outgassing. Several are available from Rust-Oleum. If you're one to worry about BPA from a water bottle, then you might want to consider them. But if you're one to lick up a ketchup spill whether it's on your painted patio table or car trunk lid, then they seem to me as overkill. Or at least that's my opinion, and I consider myself quite conservative, safety-wise, and have even been ridiculed for such on this forum.
This is all assuming you'd like to keep your unit "stock", including its waterpan. It's also true there are plenty of worthy substitutes if you prefer to "customize". My last cook was on my gas grill and used an 8" square baking pan from Walmart. Since I like water pans, my motto is to get the biggest pan that will fit in the area you've got to give it. The water gets to 212F quickly and then slowly evaporates from the surface. The larger that surface area, the more moisture/humidity you're putting into your cooker.
Hope this helps.