From front to back it's about 11.25 inches. From top to bottom counting sprak plug it's about 11 inches. Remember you need to have some extra verticle room so you can take the spark plug out. So, add 2 to 2.5 inches.
Make friends with a local backyard bicycle repairman, you'll need parts on occasion.
If your crank is not wide enough for the engine and it probably will be wide enough, you will be able to get one from him.