Through the links and advice you are looking at acheiving a 2.60 gear reduction in the speedometer gears to correct your speedometer.
You can keep the 21 tooth (red) drive gear and switch the driven gear to an 8 tooth (orange) driven gear which will give you a 2.625 ratio.
That little 0.025 ratio difference equates to .7 mile per hour at 65 miles per hour, about as much difference as having brand new tires as compared to a set of tires at the minimum tread wear amount before needing replaced.
If you are in an area similar to mine you might be hard pressed to find the speedometer gear you need, no matter how common it may be. There are online sources for the gear you need, and a simple search will put you in contact with someone who has them.