It depends on the series you are competing in. All racing cars are optimized to the competition rules.
To Wael's point, in sanctioned racing events there are clear rules which must be maintained. For example, I've seen requirements for the car to retain the full interior (door skins, dash board, center console, passenger seat, etc.) but only from the driver forward; it was legal to eliminate the rear sear interior and the trunk interior, which shaves weight.
Same goes for horsepower. Some rally championships I've participated in had mandatory restrictors on the air intake, as to set a standard for maximum engine performance in your class.
That said, if you're talking about racing for fun (at a track) outside of any sanctioning racing body's oversight, there truly are no limits aside from the time and money you wish to invest.