actually you are in luck. since it's 900MHz, you'll find planty of different antennas on ebay. just look for GSM antennas (yes - for cell phones).
it all dependes on your transmitter's output power, but i'd imagine that you could easily install tiny antenna on your RC car, and compensate for weeker signal by using higher gain antenna with your reciever.
that's how people usually do fpv - small stuff in the air/field/whatever and high gain antenna (usually directional btw) next to the monitor