Legal, yes. It can be converted to be easier to operate, depending on what limb is amputated.
Likely it would be easier to sell an older air cooled 911 and get a modern 911 with a PDK transmission.
I met this young man's parents just recently. His parents are both now retired from active duty. Their son is considered a double amputee as you will see in the video.
https://www.hagerty.com/media/videos/3-pedals-no-problem-wounded-war-vet-never-stops-driving-why-i-drive/