Nerve Damage to Abdomen causing lack of muscle tone - advise required

Hi
The issue I have is that when I was 18 (over 30 years ago) I had two Harrington Rods attached to my spine to cure a Scoliosis and a Kyphosis which was aggravated by Ankylosing Spondylitis.

I had two operations to straighten the spine - the first was an anterior release and the second was attachment of the rods. The anterior release caused the nerve damage to the abdominal muscles on the left side. I have not worried too much about it over the years, as I have always been rather skinny due to Cronhn's, but recently I have managed to put on weight due to new drug treatments and now I have an issue. I now have a small paunch which is about double the size on the left due to the lack of muscle tone.

Because the spine does not bend from a mid point of my shoulders down to my pelvis, I cannot do the usual exercises to strengthen the Abdomen, but I can at least tense the muscles on the right keeping them superficially firm (I am prone to 'pulling' muscles in my side and lower back when i do any physical labour such as digging in the garden).

Can anyone suggest how I can a) generally strengthen my core muscles - front and back so I can enjoy my garden more without causing injury and b) how to particularly strengthen my stomach muscles on the left had side so it doesn't look like I am sporting a hernia or stoma under my shirt?

Thanks for any suggestions.
Brimelow