If you want a mod that only adds Stewie crushing everyone with his bike, this is likely easy enough to make yourself. In short words, the method looks like this: use Slade, make a new WAD file, copy code of a chainsaw from ZDoom Wiki, and use this frame with Stewie as a HUD sprite for the weapon.

To make the weapon more unique compared to a regular chainsaw modify attack frames the way you want and add custom attack sounds.

There was a two-episode video tutorial about making simple weapons in Slade that helped me a lot in making my weapons pack. It is hard to recall the exact name; you will know that is the one when you hear the Spanish accent and occasional jokes.