There really isn't a difference between a negative or positive rise stem, except for maybe that the logo is upside down. I would suggest using a positive rise stem (flipping over a negative stem) with a longer length than what you currently have. Bullhorns will also help, but don't use pursuit bars (like the Nitto RB-021), because they have a significant drop, and I wouldn't do a chop and flop (cutting the drop bars and flipping them over) because the reach will be really short and won't help on a frame that is already too small. A good choice for bullhorns would be the Nitto RB-018.