Replacing your chainring with a larger one will raise your low gears (what you are calling the upper end) so that's not a solution. You would have to find a 7 speed cassette with a 12 or 13 tooth small cog. A 13 tooth would get you 7.5% faster before you spin out, a 12 tooth (not sure if available) 16.7% faster. However, with your current high gear you should be able to go over 23mph at only 100 rpm, 28mph at 120 rpm, which is hardly spinning out. It would be extremely difficult to attain and maintain that kind of speed on that bike on the level.