There are several great kosher restaurants in Budapest. They are mostly there in the seventh district in the surroundings of the Dohany Utcai synagogue, so that is where you should walk around if you want to bump into a restaurant while walking.
If you want to target a certain restaurant and go there, here are some of the best and most popular kosher restaurants in Budapest:
Tel Aviv Café
Carmel Restaurant
Kosher Deli
Hanna Garden Kosher Bistro
Macesz Bistro