If you want to seal the oven… say for 1/2 a year or so, simply spray the oven with an organic sealer. It will last for about 1/2 a year before naturally breaking down.
You can reseal the oven, or give the oven / shell a good scrubbing w/ a hard bristle brush and an organic cleaner. Once it’s been dry for about a week, you can apply a veneer… or coat the stucco shell with 3 or 4 layers of OUTDOOR latex paint.
Paint does look good - And you can apply a new coat each spring and your oven will look like new every year!