Should be a piece of cake if you match the dimensions and angles, especially if you're going to replace the roof anyways.
I had a very old three sided shed/horse stall made of old barn wood planks and a single slope metal roof. Added the fourth side with a gate, cut a gate/door into another side so I could pass through if I needed to. Some wiring, shelves and yard tool hangars and I was done. No floor, just the previous dirt.
If you're talking about tearing that down, I wouldn't. I'd keep it as a tool shed/workshop.
I had a very old three sided shed/horse stall made of old barn wood planks and a single slope metal roof. Added the fourth side with a gate, cut a gate/door into another side so I could pass through if I needed to. Some wiring, shelves and yard tool hangars and I was done. No floor, just the previous dirt.