[blndebmbr]

I completely disagree^^^^

You bought a used bike so you have no idea how long that piston has been in there. I replace the top end in my 80 AT LEAST once a year. I would def jump on it if it is brand new!!! It doesn't hurt to have one on hand, especially if you ride hard. (Mine is a race AND a recreation bike...lots of hours in different climates) You may as well see if you can't get some rings and seals on hand too. Especially with the 125's...most people ride the piss out of them. I would bet that top end is going to need a rebuild sooner than later anyway. If it's not kicking your ass when it hits...odds are it's going to go.
I'd buy the spares if it were me.