Welcome.
As Geoff has mentioned, you need to be more specific in stating your location in your profile. What is required is highlighted for you
in red and there is no juistification for not complying; your cooperation in this matter is greatly appreciated.
OK, you mentioned that you have a D70 with a 70-300. There are three different Nikkor 70-300 lenses, plus a Stigma, and, IIRC, a Tamron. Which of these do you have?
For motorsports, I'd strongly suggest either the 80-400VR or the 70-200 + TC 1.7. Nothing less will really be acceptable, and I've lost count of the number of people who have, for instance, bought the Bigma, only to then go and buy the Nikkor that they should have bought in the first instance.
But it's also important to learn how to use your equipment correctly: why do you believe that the 70-300 isn't up to the job? I agree that it's probably not, but I'd like to understand your reasoning.
And of course what sort of motorsports are you shooting? Technique plays a very large part, and the lenses are only a means to an end.