Will a suzuki lt 185 cylinder and piston fit a lt 125?

Yes and no . The LT185 cylinder will bolt right on to the LT125 and DRZ125 bottom ends. However it will not produce a great deal of power due to the fact that the LT185 piston has a very low dome compared to the LT125 & DRZ125 . This leaves you with a very low compression ratio and a low power output.