Keep it under 100,000 polygons, rather target 50,000. The track geometry should still be as simple as in the original games, however, slopes and other details like buildings can have a higher polycount to work out more detais.
The majority of the complexity should be spent on details like plants, buildings and key elements that are easily visible.
Tracks should have more details in terms of geometry and textures. Certain design elements can be added to give the tracks more atmosphere.
The raceline should not be changed much, however, rough parts in the geometry should be smoothened out to make the racing experience better.
Generally, there is more artistic freedom on tracks than there is on cars.