Don't turn the engine over! Gently brush or pick away all the dirt you can see. Inspect the outside of the engine for cracks or other damage. I would take off the carb and soak it with water to get the remaining dirt out. I then would take rearplate off the crankcase and the head of the engine and flush with Varsol or WD-40 then inspect the inside of the engine. Only after you are sure the dirt is all removed would I oil up the engine, re-assemble and start it.