bizzyaya:
What is a good replacement for Kempo in the 90 day routine?
I don't suggest doing Plyo twice in one week. A workout that intense coupled with another intense legs routine is really all you need to hammer your lower body. Overtraining those areas, especially on consecutive days, are a sure ticket to the sideline, if not now then eventually.
If you're not finding Kenpo X as challenging for you as prescribed, then you also have the option of looking into Kenpo Cardio + or any of the interval training discs from Tony's One on One series. Or you could supplement the Kenpo workout with a bike ride, jog, hike- something to get your heart rate elevated.
Best of luck,
Kim