Take a look at this year's entry list in the historic F1 Championship: http://www.tgpf1.com/home.htm Not one Ferrari. How come? Please don't tell me, that this is the outcome of the "Clienti Corsi" programme and Ferrari prohibits its cars from participating.
In the U.S., Bud Moeller still runs his 312 T5 (the only F1 car I've ever sat in!) in Historic Grand Prix events. http://www.historicgrandprix.com/home.htm
I suspect there is also an element of parts & maintenance costs - there are so many cars with Cosworth engines Hewland gearboxes Lockheed/Girling brakes etc etc that keeping them going is a relatively easy task
If I understand correctly, before the Clienti Corse program, Ferrari was known for being particularly terrible with doing repairs, providing parts or even information for their older F1 cars so many people choose to run something that will not be so difficult to fix.
