Unfortunately only 2 chapters of Kimi no Tame ni Hiku Chopin have been scanlated, But I Instantly fell in love with the cute characters and the sweet story.

I am looking for a series that also shares a sweet and honest main heroine and a nice simple plot. Probably a shoujo title. No tragedies please, just a really light-hearted story with some romance
Also art is important me too. I am a fan of art styles with clean lines - no sketchy lines like many older manga.

La Corda D'Oro is very similar. The main characters are very much alike and there even is an invisible musical companion. I'm pretty sure it's completely scanlated, plus it's very good :]

"Nodame Cantabile". Its very, very similar to this storyline and is actually considered a classic music manga!! The art is very simple, nothing elaborate or amazing, but the story and journey of the two main characters while they progress as piano players is something worth reading. It is a shoujo btw.

There is also "Piano no Mori". Same concept of finding a voice withing one's own playing however there are more realistic elements to it and is a Seinen, making it more heavy in terms of content. Its still an incredibly good read regardless and I actually prefer Piano no Mori over Nodame Cantabile. The art style is a little sketchy, but it's modern, so I don't think you'll mind all that much.

These are the only two I could think off the top of my head. I hope you enjoy them!!