There are a few more software I2C libraries: https://github.com/Testato/SoftwareWire/wiki/Arduino-I2C-libraries.
Sometimes a few code lines in the sketch are enough to change to a software I2C library, sometimes you have to change all the library code.
The Mega has so many pins, and you happen to use pin 20 and 21 for something else ?
It is possible to use the SCL pin for other things when the I2C bus is not busy.