《朗文当代高级英语辞典》第6版 指出：不要说 someone "has difficulty to do something" ... 而要说 someone  has difficulty doing something  or  has difficulty in doing something. 

      所以：

      The composer has difficulty writing this piece.  

      这句里面writing 不可以改为不定式 to write 。

不可以，have difficulty (in) doing sth.“作某事很难（很费劲；很麻烦）”是常用短语。又如：

I have difficulty (in) remembering names. 我不易记住人名。

We had no difficulty (in) finding the house.  我们毫不费劲地找到了那所房子。

不可以.