Wow, those must be some big hills! My zx50 would slow to about 35mph at the most about the steepest hill around here, which is actually a bridge, when it was stock! It sounds like you have done about all you can do though without doing the big bore. I think about the only thing left would be the carb. You might be able to upjet the stock carb. I never tried on mine. I just did the big bore and a 19mm carb! It doesn't even notice hills now!