Have a look at the Classic Sports Car Cub's "Swinging Sixties" series,
http://www.classicsportscarclub.co.uk/
http://www.classicsportscarclub.co.u...ginsixties.htm
40 minute races for one or two drivers with a compulsory pit stop. Good VfM & very popular. Sounds like it would suit your modified racer, although note that the silhouette has to be the same as a production car in all views and the engine type has to be the same.