The concept of "stomach" can be expressed by simply patting your stomach twice.
Or use fingertips of a "bent-B" hand to poke it twice.
Or you can point to it.
Remember "signs" are not English words.
If you have a sentence like "I can't stomach the sight of blood" you should use a sign that represents the concept. For example "tolerate" (a version of the sign PATIENT) would be a good choice.
If you are in a doctor's office and need to describe the stomach in more detail you can use "classifiers" to describe the size, shape, and location of the stomach.
Notes: