Buy a set of 215/85R16 tires on it. They are 30.5" compared to your 28" tires. That will give you a noticeable increase in speed with less RPM.
Have your rims sandblasted and epoxy primed, and use epoxy on the rivets which hold the rims together. You can then run those tires tubeless. I recommend the valve stems which are held on by a nut.
