I read this book as a young adult in the early 1980s, so it must be older than that. The book is NOT From The Mixed Up Files of Mrs. Basil E. Frankweiler.

As I recall it, there is a character who lives a modest life in New York City. The character is a passionate painter. He/she meticulously recreates great masterpieces, then from time to time sneaks into the Metropolitan Museum of Art (or possibly some other museum) and replaces an original with a forgery. Over the years, he/she has done this a dozen times, but no one ever discovers that the paintings are forgeries. The originals then sit in his/her modest apartment. The forger makes no money off his/her expert forging talents. A few of the forger’s friends know he/she does this, but there is never any sense that the forger will get caught.

This character may not be the main character of the book. I seem to remember that the book had an ensemble of unusual characters.

It is possible that the forger character, or another character, lived in Central Park.

This is probably a young adult book.

Thanks!