When I modify, I usually first look at what elements comprise the move--- here there are lunges, explosiveness, and changing which foot is in front--- and focus on one or two of them that I can do. So for explosive lunges, I either do the plyo jumps, but don't switch foot positions (so do a certain number with right foot forward, and the same number with left foot forward, stepping into the change), or I do the 'lunge and foot change' combo, but take out the plyo aspect, just stepping into position. Or you could just do lunge in place, changing the front foot 1/2-way through.
If even lunges are tough, try them while holding onto the back of a chair, or use a body bar or a dowel for help with balance.