>Firstly you wouldn't be able to have anything like drifting
Sonic doesn't need a vehicle for drifting. It will look comical for ponies to be "drifing" but if you think about it cartoon wise it's likely to see such thing, well maybe from Pinkie Pie or Rainbow Dash.
>Secondly, how would you even balance that?
Why can't each pony have various levels of speed, cornering, boost and etc? What some ponies lack in some stats they could make up with others, for example if Flutershy is not as fast as Rainbow Dash, she could be better at cornering than she is.
>How could you make sure a character who can fly faster than the speed of sound has an equal chance of winning as a character who's scared of her own shadow?
This is the same character who can't out-run Pinkie Pie and while ground-bound can rival Applejack. Also while Fluttershy is not known for her speed, it is with Rainbow Dash she can save Rarity and Spike as well as pull western carts at great speed.
As for flight, magic and power, why must they be restricted? In the Sonic Riders series, Speed, Power and Flight were used for shirt cuts, why not shotr-cuts for unicorns, pegasi and earth ponies?
Also if you recall in S&SASR that Sonic ditched his car when he went Super-Sonic, pressuming by name sake he is faster than the average Sonic Boom, Sonic Boom being something Rainbow Dahs use sparingly, in other words why can't "Ponykart" have All-Star moves?
If balancing stats will still be a problem, why not throw in a "Racing Accessory Shop"? It works for Team Fortress 2 when you can buy accessories that alter a characters skills in gameplay.