Too big? Naaaah!
Depends on what you want to do??? On a sprint track it won’t beat a good 125 in the same chassis, but they’re a blast to drive!!!
I build Cobalt Superkart chassis, and I put the 450’s exactly in the same place as the 250’s, and they balance perfect. Balance left to right is pretty important, but that being said if it’s a lot easier to mount the motor offset to get the seat to work, then you can adjust around it.
A CRF250 would b a better match for a 125 chassis…
CR