The Warbler can be equally easy (if not even easier) modified into a 40m WSPR transceiver.
That what needs to be done:
- use 7.040MHz crystals for the filters
- use a 7.038MHz crystal for the oscillator
- pull the oscillator frequency to 7.0386Mhz
- adapt the PA and low pass filter to 40m
One could consider using the sound cards audio signal to key the transmitter, there are simple solutions out there. See also Dave's PSK20, which is audio keyed. Documentation on Dave's webpage.