I believe you have to have some sort of front seats. Driver and Passenger, but no need for the rear seats.
I have seen some of SM cars also change their steering wheel to a quick release one and you can gut out the interior of carpets and such.
Stereos, aren't mentioned as a no-no, So I don't see why you can't take those out as well. ...
I did find out though have to stay in a minimum weight of 1800lbs. But with a focus, I dont think you have to worry about that.
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R.I.P - FELLOW FF MEMBER, IZZY, BLK2k3GTS, will always be remembered. We will compete on a different course again one day.