A bit late but I didn't see any answers for this question.
It is rather trivial to set video bitrate (say, 64 kbit/s) with NReco VideoConverter component:
var ffmpeg = new NReco.VideoConverter.FFMpegConverter();
ffmpeg.ConvertMedia("input.avi", null, "output.mp4", null,
new ConvertSettings() {
CustomOutputArgs = " -b:v 64k -bufsize 64k "
});
more options you can found in the ffmpeg documentation.