[classicchrisk]

I had planned to enter my Scalextric BMW 3.0 CSL, but don't think it is allowed any more. I expect it is classed as a Grp 4/5.

Yes you are correct, the Scalextric version BMW 3.0 CSL car did not run in the Targa Florio (as far as I am aware) if it had been I would have designated it as a Group 3 car . Save it for another day....
The Maserati, Ferrari 166 and the Jaguar D type are permitted as in Group 1. 
And the Porsche 911 Carerra RSR would be classified as a Group 3 car.

Or is the Porsche a 934, eligible for Group 2?