You have lots of options. You can go to a motorcycle shop and buy numbers for yhe plates, and they usually have the smaller ones that would work on your helmet. You would not need to cover these, they are pressure washer safe. You could also go to any sign shop and they can print you out your numbers. You can 2 or 3 digit numbers printed on one sheet, which is nice. You could also have something neat printed up for the chest protector that would fill up the entire "sticker area", like his name or something. For this you would need to measure the approximate size, then cut it out to fit.
Rich