This should help....I know it says six volts and your bike is 12v.
run a search ot the web for
"555 calculator" and "LED resistor calculator" (bill bowdens is good)
these will allow you to adjust certain values to get it running
The 555 timer is fairly robust device and will run happily on 12v
However...
The supply rail from the the alternator to the battery is subject to fluctuations , check your supply voltage! 18volts is not unheard of in an automotive / motorcycle charging circuit. The 555 will run at 15v , but any more u will be pushing your luck.