[Graham]

Just looking at what's available, will this only be for cars that actually raced rather than those that only tested for the event, I'm thinking Porsche 917K in the Targa Florio as an example.

Well the Porsche 917 is on my Targa Florio list, so I guess it may have entered but not actually run. 
Basically any car of the period which entered a International Road Race in that Event, so for the Targa Florio Event could also include prototype cars which ran in the Tour De France.