Looking for 11/28 freewheel or other higher gear option
I have an older, like new, low end mountain bike that I installed an electric hub onto to use for commuting. The problem is the bike is geared too low for street use especially with the electric motor assisting. I currently have Shimano SIS gears. The crank has 28, 38, and 48 teeth. The rear is an MF-HG37 FREEWHEEL with 14/28 gearing. What is my cheapest option to improve my gear ratios for increased speed? I really on nee the high speed in one gear, for the other gears, I will take what I get. The best solution I can find is to install a cogset with 11 teeth on the smallest cog, but they are apparently pretty rare nowadays. I am open to any and all options as long as the cost is low. My only goal is to increase my top cruising (pedaling) speed from around 18-19 mph to around 22-23 mph or more.
Note: My motor will propel the bike up to around 20 mph on level terrain, no wind. I would still like to get some exercise out of my commute which is why I am looking for a higher gearing option.