This is my first game jam, so I probably shouldn't be giving advice, but I think if something has a time restriction it'd be best to do everything (code-wise) from scratch.
Good advice! However, because of the time restriction people tend to reuse their own code or borrow code from others. As long as you state where you get the code from this is no problem, but often the code origin is not mentioned in the submissions. Since code is difficult to copyright this is probably a common practice in the gaming industry as well, but I am just guessing. In a jam I think it is ok to experiment and try new things even if this means not everything is done from scratch.