I have two 6 cyl. diesel engines one with turbo and starter motor 4KW and the other without turbo and starter motor 2.5 KW What happen if I replaced 12V 4KW with 12V 2.5KW starter motor?
From the definition of power on Wikipedia:
The output power of an electric motor is the product of the torque that the motor generates and the angular velocity of its output shaft.
So, if your two starter motors are spinning at the same angular velocity (in order to get the diesel engine started), the 4KW motor will have not quite twice the torque of the motor with the 2.5KW power rating. If that's not enough torque to turn the engine at the necessary rate, the engine won't start.
Go to AutoZone.com (or any large auto parts website) and enter the vehicle information to look up the starters- if they are the same part numbers they are compatible. I would assume if everything matches up- ie. Bolt pattern and distance to flywheel from teeth then they should be compatible. The 4kw and 2.5kw is only a difference in battery power used to spin them. They're both gonna spin enough to crank it over. It would matter only if you had one 6cyl and one 8cyl engine. Just make sure you got a good strong battery on there- or a jump box.