28 YEARS ??????????????????????? NO POWER UP YOU SHOULD CHANGE THE TIME FRAME IT MAKES NO SENSE. SORRY TO SAY THIS BUT I DONT BELIEVE YOU HAVE EVER WATCHED OR READ THE DBZ MANGA.
Kind of small improvement in 28 years , i mean he has access to the dragon balls from namek , could wish for gravity chamber , perfect training ground , even an infinite ki gen (just some examples), or he could try to master Oozaru to try ikari , it is 28 YEARS ...