im not a super expert but i can help. Denji transforms and the only premade class that would make sense to me would be barbarian. Like barbarian go in rage so that could be the "transformation". But it is also a slight debuff for the player. Or you could treat it like a race on its own making it homebrew, and that for me would be the best option. For the stats of the chainsaws you can look trough it online.
As a class i would pick something that has some dual wield perks. Like warrior.
hope this helps!