Being that its a private event theres not a whole lot you can do legaly and might i ask why you would? From the content of your post and the limited use of motor vehicals sounds to me like the cars trucks etc are serving only as a meathod for moving equipment etc around the site.

Being that its pre 17th century my guess is theres lot of heavy equipment like various forms of catapolts and other seige engines being used. What you might be able to do is sugest to them to use bike + trailer to move small stuff period weapons clothing etc instead of useing a car that will tend to make more noise and spoil the atmosphere.