Let me say at the outset, I've never rehabilitated an orphaned woodchuck or any other marmot, nor have I kept any as pets. However, I do know a bit about marmots and people have asked for suggestions on how to care for orphans. I'm reproducing advice from those that have kept them as pets. People have told me that they've fed orphans with goat's milk and bread soaked in cow's milk. I'm not an animal nutritionist, but they both seem like a good start.
Marmots are generalist herbivores.
Start them on fresh fruits and vegetables as soon as possible. Marmots love dandelions and cow parsnip. If they're getting fresh vegetables, they'll probably not need water. Marmots are often infested with mites and fleas. Be careful not to poison them though.