Difficult, seeing that OTL Nogorod Republic mostly held the barren wastes of northern Russia, while Muscovy held the fertile plains of eastern Europe. Plus, by the XV century it mostly hunted out, denying Novogorod its primary source of income. AND on top of that, if Novogorod was to survive, it would have to deal with the Swedish Empire, which would propably be too much for it to handle.
You would need a quite early POD, one which allows Novogorod to expand south early on, it's republican system attracting some local dukes and joining with it, rather than being annexed by Muscovy or any other central-russian power. Destroying Muscovy by the Mongols, and somehow keeping Poland and Lithuania out of the Rus would be the simplest way to go.