Ignore previous answer... ET4 are cracking bikes as is the ET2.
Generally you will get decent answers here.
Is it a two stroke engine or the 4 stroke.
If it is the four stroke then I am pretty sure you can but you will need to change all the other things, carbs filters etc etc.
I would chuck in the ET2 motor and then sell it and then buy a ET4. ET2 you wil get a decent price for.
BB, August 2009