Hi Brocky,
Painting is your choice, if you are going to use the mower just give it a general tidy up. The surface oxidisation will come off with a bit of rubbing using a scotch pad, that's the green ones that you use on the pots. To clean the exterior use some spray can type degreaser and a brush or just plain kerosene.
Be careful not to get water near the carb as it finds its way in through the primer pin and ends up in the engine if you pull the motor over. Generally a good idea to clean out the carb as well because they usually have old fuel and corrosion in the fuel bowl. When removing the fuel bowl if the gasket isn't broken already just loosen the bottom nut and with a spanner or the like tap all around the bowl near the gasket until it loosens.
If you just pull the bowl off the gasket will break for sure and they are not easy to get.
