Rees,
I went through more than 500 John Johnson's in the Illinois Data base. Tried alternate spellings, etc, and I could not find anyone who fit Mr. Johnson's profile in terms of age, heighth, nativity, etc. I think if he served in Illinois, he used another name perhaps or perhaps he served in a unit from a neighboring state. Do you suppose he was a "galvanized" yankee and served in the regular army in the west?
FC