Most track days require that you have a number on your bike,
at least on the rear sides and the front.
For a casual track day you can do this very informally by just cutting strips of
duct tape or black electrical tape.
Note that, conventionally, black numbers indicate a pro racer, while red
numbers are for amateurs. Your track day might not enforce these rules, but try for red
if possible, to avoid confusion (and not look pretentious).
I made these numbers by printing a template on my computer then tracing onto a sheet of peel-and-stick vinyl. This let me customize the size to just fit in the available space.
I have them permanently stuck to a second seat cowl for easy change-over.
Of course you can stick them to the original cowl and remove them after (or
leave them on the bike, for a cool appearance).