Moving a large amount of kit can be one of the more frustrating parts of reenacting, especially when traveling overseas. You need to pay or find ways around high transport costs, while packaging everything securely.
Airline regulations are varied, but most charge regular or oversized baggage fees according to the number of linear or dimensional inches (length plus width plus depth). Expect to pay surcharges in the area of $100 for large shields and long spears. A gorytos with a modicum of flexibility can be crammed into your suitcase. US Airways requires items to be packaged in hard-sided cases (I've found that a rifle case works best for a recurved bow and arrows, and can hold sidearms as well; US Air doesn't consider this oversized). American Airlines doesn't have such a requirement, but does set an upper limit of 75 inches in length which should be just barely enough for a 6-foot spear. United Airlines requires documentation for weapons, but does have a separate cargo service and special hard-sided boxes in very large sizes for an extra charge.
Shelley Powers of Hoplologia informs me that rather than checking their spears and shields as airline baggage, the Plataians got theirs to Marathon 2011 by commercial shippers, which is viable for large groups. She (and Ashley Holt of the Hoplites Association) also recommended plastic pipes for spears. Shields were packed in bicycle boxes.
Since your kit will take up so much room, travel as lightly as possible otherwise. Bring only as many changes of clothes as you need, and travel-sized toiletries (don't forget mosquito repellent and sunblock). Wear a single jacket with plenty of pockets, and pick one pair of good walking shoes, with buckles or velcro so you can take them off quickly in the security check.
Your schedule will be busy and unpredictable, so eat well at breakfast and lunch. If you're not camping out, or not camping out for the entire event, I can personally recommend the Thomas Beach Hotel in Nea Makri, which is a short drive from Schinias Beach where Marathon 2011 was held.
Next up: A proposed rulebook for XMFM the reenactment group.