I am used to songs, like hymns written in 4-part harmony, where unless otherwise indicated, the top notes are the main melody.

Classical piano pieces are not really songs, so is there a melody line (when it is not obvious)? Should I assume the top notes are main melody?

For example, this section from Fur Elise:


Should I bring the top notes out as main melody? Or should I play about equal strength for all notes?

TIA!

Usually to find where a melody lies, you need to just use your ear to decide where the strongest melodic line is. In Fur Elise, play with a the top note a little harder during this section. I believe that, in general, where you're playing chords that the top note is typically the melody line. Of course this isn't always the case, but usually that's what composers want since the top voice is the most easily heard. In the example, the top is definitely the melody.

Yes top notes are the melody on the piece you show.You can emphasise the very top note slightly. Like much of Beethovens piano works the left hand acts as a pulsing accompanyment.
Some classical pieces translate well into songs.
In fact many many many songs steal elements from classical works.
The melody can occur anywhere either bass or C clef but usually its found in the C clef.

Don't you mean the G clef? I've never heard any of the piano clefs referred to as the C clef....I've only heard alto clef called that.
But yes, usually the melody is in the treble clef, but certainly not always. Never assume so with Bach or any contrapuntal work. Even a lot of regular piano works often transfer the melody to the left hand for a time.