Ahoj Davide !!!
I've just analised all things you would have to make.Here is a shot with almost all corrections for converting Ju88A-4 into C-6 version.
Unfortunately,there haven't beeen enough space to put all my ideas.Therefore, I've been forced to write the "short" description.
The first thing you have to do is to remove the bomb racks from lower surfaces of wings.The places where they were mounted should be tread with aputty and then smooth.
A) The second correction is to remove the "glass" nose.It should be replaced by a new one that you could make by yourself.How to do it in an easy way?To be honest I have no idea.The easiest way is to "borrow" the part from an other model,byt I don't know if it is possible to find it.
Usually,I make this kind of elements using a sheet of plastic and a wooden model.The plastic is warmed up to be soften and then it is stretched on the model.After it had been cooled down it could be cut off and make fitted.Certainly,you should be careful you don't burn your hands.
A1) In addition,you have to remove the small windows from both sides of the fuselage.Put there small pieces of plastic or cutting the "glass" nose off leave the pieces of the old nose.Of course,the putty is needed to make the area of the fuselage smooth.
B) Spinners and propellers.The C-6 version was equipped with the broad chord VS-11 propellers and the ones of your model seem to be O.K.As far as the spinners are concerned,they seem to have incorrect shape.According to drawings they should be more sharp in shape.
C) Looking at the pic of the C-6, I've posted earlier,we can see that on its exhausts were mounted flame dampening exhaust shrouds.These elements have to be scratch-built by you too.What material you'll use it depends on you only.In my opinion the aluminium plate of a can of beer could be good (also beer).
D) You need add two FuG 101 radio altimeter antennas.How they look and where they should be fixed you can see at the drowing.
E) The old FuBI 2 antenna and its racks should be deleted and the new one should be repositioned and fixed at a proper place( according to drawings).Remember that the new antenna was covered with a kind of plastic covering so you can use a transparent piece of plastic and paint it in the way you can see at the second pic of the C-6 I've posted.
F) Check a fin and a rudder.The fin of the C-6 version had different plate panels than A-4( you can correct them if you want).The rudder of the C-6 seems to have a metal covering when A-4 had been covered with cloth.But it is a detail only.Your model seems to have both the fin and the rudder correct.
G) Upper surface of the C-6 fuselage.There were :
1) FZ-6DF antenna which seems to be covered with a transparent covering with metal strips on it.It should be easy to make.
2) A life raft compartment ( if you want you can make it open.In the case you have to have all interior of the bay and a life raft scratch-built.Besides, you have to add the life raft tow line as shown (4)
3) FuBI 2 antenna - it was depicted at E) point.
4) The life raft tow line.
H) The bugled conopy with two MG81J 7.92 MGs in armoured turrets.The pic of your model shows that the cockpit conopy is correct.It will be enough to add a modified antenna and lead-in wires( as you can see in A and A1 drawings).Besides you have paint the cockpit conopy frame according to these drawings.(In my opinion your conopy was painted incorrect.There is lack of the first strip of the frame.)
I hope I was helpful for you.These corrections are quite easy to make them ( except the new nose cap).Of course, the putty,files,a saw-razor-blade ( the Czech invention as my memory serves) and sandpaper will be needed.
That is all.To do or not to do - the choice is yours.