Replace a cross brace with knee braces?
Yes, if...


The means by which you fasten the ends of the knee braces to your structure is adequate. It must be very rigid so no slipping or flexing can occur. There will be substantial shear and tension forces at play. Your mortises should fit snugly.
The posts in your structure are rigid enough. They'll carry a significant amount of bending force due to the longer torque arm that the shorter braces create. Just how much depends on the ratios of braced to unbraced length you go with. I probably wouldn't do less than 1:3.