Short noses can still search
Dogs with flat faces have a shorter nose, a squashed skull, and often narrower airways. Some have a condition often called brachycephalic airway syndrome, which makes breathing harder. Your vet can tell you whether your dog is affected and how much exercise is safe.
Even so, these dogs have noses and love to use them. Smelling does not require running. A Pug on a mat with a handful of treats can be a keen searcher. The main change is to keep the work easy on the body.
Some small breeds are bred for scent work too. A Beagle or Dachshund has a great nose, even though they are small. See best breeds for scent work for more.