Not that i know.

Start as tribal Novgorod, move capital to coastal county and reform as merchant republic, done.

引用自 neidhard：

Does anyone know any mod that allows you to play as the Novgorod republic (preferably updated at least 5 years ago) or coverts it into a monarchy? thank you in advance!!

Though there is still no single Novgorod-enabling mod, the HIP mod has a short period where it is a republic,but while you can't select it to start, when i selected a feudal vassal first for some reason i was in-game then able to console-switch to that republic and not get a game over and play on.

Link to mod download :
https://forum.paradoxplaza.com/forum/threads/historical-immersion-project-download-faq.748775/page-245#post-29602852
There is also an invitation in that post for the discord 'server' of HIP.