I have done this with many engines. Remove the carb . Cook for about 2 hours and start checking. Some come clean in 2 hours, some 4-5 hours. Wouldn't leave it overnight. Remove the glow plug and turn it over a few times submerged whrn you first put it in. Careful when you remove the engine from the hot antifreeze.. It is really hot and retains the temperature for a long time. Shake out anti freeze, add afterrun or marvel mystery oil. Will look and run like new.

John