I don't think they provide decals, you should purchase numbers yourself and have them on the bike prior to the event. Most bike shops have the 5 or 6 inch numbers in black or white. Probably online places as well. If you want to get fancy, you can contact places like dcgraphics for some fancy fonts and colors, otherwise, the "generic" numbers work just fine. If it's a bike you use for both street and track, it helps to get some white vinyl and make a number plate, then stick the numbers on that. Instead of peeling the backing off the number plate, just use duct tape all around the edges to tape it over the headlight. Then you can remove it and reuse it. That's what I did when I had a bike that did street and track. Tapeworks has vinyl in bulk, you'd just need to cut it so it fits reasonably well.
If you don't get numbers in time, you can always make numbers out of electrical tape. Just so long as they're readable, that's fine. I've seen folks make numbers with blue painters tape, too. Cover the headlight in a light colored duct tape, then put numbers on with electrical tape.