I've seen people as both a mechanic and carpenter and suspect they were used somewhat interchangeably. Perhaps the modern version would be a building contractor or handyman?
The National Park Service has a website where they've indexed most of the service records. Here is the link:
There are a few possibilities:
1. James Isbell, 1st Sgt. Company A, 12th MS Cavalry
2. James B. Isbell, Capt. Co. H, 12th MS Cavalry (further research shows this man was in his 20s during the war). This may be the same man as #1 above.
3. J.J. Isabell, Company A, 14th Consolidated MS Inf.
4. J.J. Isabell, Co. B, 6th MS Infantry from Rankin County. They were consolidated with other regiments to form the 14th MS in the last few months of the war. #3 and #4 are possibly the same men.
5. J.S. Isbell, private, Co. A, 6th MS Infantry.
You could send off for the service records of these men which would show if they were detailed in some manufacturing capacity in lieu of regular service. Given his age (and if he served) your James Isbell would probably have been in a State Troops (local homeguard) unit so I'm not really confident that any of these that I listed are your man. You never know, though, so it's worth a shot.
If he filed for a Confederate soldier's pension (check with the Mississippi Archives), it may offer additional information as well. His widow may have also filed based on his service so check under her name as well.