Personally, I'd try to avoid stripping the engine down if possible, but it depends on how far in the dirt got.

I've dunked an OS 25LA in the dirt a couple of times, all I did was to brush off the dirt from the outside and remove the carb.

I washed the carb out with denatured alcohol. The one thing I forgot, and what you need to do, is to take off the remote needle assembly and rinse that through too. After washing the parts out, use a can of compressed air (dust remover from Radio Shack or smilar) to blow through the needle valve assembly and carb jet.

If you washed it off before removing the muffler, no dirt should have got into the exhaust area, so you shouldn't need to remove the head.

You might want to take the plastic back plate off and see if any dirt got into the bottom of the engine, but I'd try not to interfere with the piston/liner if possible.