I did this mod today, replaced C20 with a 10uF electrolytic cap. One note to anyone who might consider doing this, I found I had to lower the tuning voltage at TP1 in order to get clean sounding audio at the low end of the FM band.
Without lowering the voltage, the audio at the low end of the band sounded like it did when C19 was installed.
The was I tuned was as follows: After replacing C20, I tuned to 107.9 and aligned the VCO for 9.0v at TP1. Set the unit for 88.1. Played a song with a lot of bass and adjusted the VCO counter clockwise until the audio cleared up. Reset the transmitter to 107.9 and verfied that the PLL locked and confirmed with a frequency counter that the transmitter was on frequency.
Retuning may or may not be necessary as each unit is different, but just thought I'd pass that along incase anyone is pondering whether or not to change C20 or has done so and was disappointed with poor audio on the low end. Check the tuning...