It really depends on how rusty it is.
Like an idiot, i had an S250 outside for a while with a tarp over it. It didn't cover all that well and let water into the back over the radiator and oil cooler. It made it rust and when i started it up, oil poured out the cooler all over the engine bay. Hopefully yours isn't that bad. I spent quite a few hours silver soldering up the holes.
On others, I attacked it with my sand blaster to remove the rust and give it a nice thick coat of black paint. They never rusted again.
If you have access to a sand blaster it works a treat, with a wire wheel or brush, you will never get all the way down to get rid of the rust. Even a hand held blaster would work, but it will take you some time to get it all done.
