I bought my 550 without the starter button. When it came time to start it for the first time I crammed the end of a plastic pen in the hole. Worked fine. After all the carb cleaning and valve adjusting and cam chain tensioning and timing and carb syncing it starts on the kicker without fail (Well, almost. Now that the weather is cooling down I'm having to puzzle out the 50 degree starting procedure.) So, don't let a missing starter button stop you, grab the nearest Bic and crank that thing up.
Good Luck,
~john