Changing the wheel circumference - should give you a potentially more accurate speed, or if you don't care about accuracy, you maybe able to get an extra mph out of your bike before the speed restriction kicks in.
Can you fit it yourself - I'd say in general yes, if your a little handy with basic tools. The complexity will come from whether you have to take the crank arms of your bike, to get the motor cover off which could require a crank puller tool. I'd look at it it like this, fitting the actual Kiox and running the cable down to the motor is fairly straight forward, it's just how easy will it be for you to get your motor cover off to plug it in...
(Tip: I searched posts on here regarding my bike and removing the motor cover to get an idea for how easy it would be for me)