First what makes you think to replace the evaporator? Just cause they cant find a leak doesn't mean it is an evaporator, tis isnt something you want to replace just to replace it. When I have one that has a very small leak I charge the system and leave it in an enclosed area overnite so I can detect the leak. Also if it is a euro model what kind of compressor was installed on it, some are a sanden ( hope I spelled right) and some are regular a-6 compressors ( big long one down below on drivers side. If it is an a-6 these dont work well with 134 conversions. There is no aftermarket drier or evaporator to make it cool better. My suggestion would be to find the leak ,fix it.
Then depending on what compressor it uses , if it is sanden it should cool ok with 134, if it is a-6 go back with r-12 , I know its expensive but if you want it to cool correctly thats what you do. Good luck