I doubt that a single PI section low pass filter can achieve the -43 dB 2nd harmonic suppression that is required by the current FCC regulations.
Many old designs suffer from that same limitation.  There is no longer an exemption for QRP transmitters.
The original Pixie was OK because it was built before that regulation was in place.  But all transmitters built after January 1, 2003 must meet that requirement.  See FCC part 97.307(d).

73,
Don W3FPR