Modern tyres, brake materials, suspension, porting, pipes, ignitions, tuning methods... there's no doubt that the typical 1974 model bike in 2008 is a heap faster than it was in 1974.
Whether the best guys riding them in 2008 are as fast as the very best in 1974 is another question - I'd speculate that the answer is 'yes', although I'll also accept that I haven't seen a recent world champ on a pre-75 VMX bike.