Protein rich foods will help the healing process - chicken, red meat, eggs. Teeny bits of whatever you are having for dinner will be fine, you don't need to buy anything special.
Obviously stick to plain meat/egg sources, bacon and sausages have too much salt, a plain boring bland cooked egg, piece of beef or chicken with nothing added is perfect.
I give my ill rodents manuka honey with a high UMF. It has anti-bacterial properties which help the body heal. Its also a great energy boost.
My russian hamster, Dash, had a bad leg break when he was about a year old. It was a clean break of the back leg - the main bone in his thigh snapped completely, but neatly in half. His skin remained intact. The vet took an x-ray, but said there wasn't they could do. Cone would be of no benefit to the hamster, any splints would be chewed off, and an operation to amputate would be too risky. He did admit to being unfamiliar with treating hamsters, I do wonder sometimes if my present vet (who has lots experience with rodents) would have considered an amputation.
Dash was treated purely with cage rest - I kept him in a much smaller cage with no wheel for about 3 or 4 weeks. His toys and hides were changed daily and he was handled daily to make sure he was well stimulated. His leg healed up, although always looked a bit wonky - the bones overlapped during the healing process, giving him a very thick thigh. He had a habitat ovo set up, and for the rest of his life it was set up to he couldn't climb - we didn't want him to stress out the weak leg. He had loads of floor space. He lived to be three and a half - good going for a russian hamster