Not too worried about the bottom end lol
I've seen some Ninja 300 builds, big bore with high comp pistons getting up into the low 60 hp range. The bottom ends on those bikes don't last at all. They need a complete bottom end rebuild after 1 season.
Kinda hard to say though what a Z125 can take long term. I'm sure the 143's and 155's run forever. Good luck with it. At the worst, well at least these bikes are cheap! Beats blowing up a $12K supersport.