I know what a 303 is, and it's even mentioned in the episode. But one would be "repen" an area code, not an instrument, and the idea of the instrument makes even less sense in context:
here i am im a nerd who cares...didnt choose the thug life and it didnt choose me
i ant repen on no 303 im sitting down playing on my ps3 hopin to live life to fulfil my dreams
i dont got streetg cred but ill beat you dead at mtg
MisterMonstur is contrasting his nerdy lifestyle with the expected trappings of "the thug life" and "streetg cred". If we understand "303" as an intended reference to Detroit, his statement is that he is no hardscrabble B-Rabbit, merely a PS3 gamer who happens to rap. If "303" is a reference to the bass synthesizer, either he is inserting an unrelated detail about music production into an otherwise thematically straightforward set of bars, or he is saying as obliquely as he can that he can keep time without a beat, even though the surrounding diction is numbingly direct (and still thematically unrelated). The issue is not whether MisterMonstur is knowledgeable enough or not to rap about Roland gear; he simply has little reason to do so and less to do it in this way.