You'll find a LTC3108 model in LTSpice, probably the only way to simulate the device with a detailed model.
If you study the LTC3108 datasheet and block diagram, you'll understand why it hasn't a feedback pin. It supports fixed programmable voltage levels.
Generally MPP can only work if your system has a variable load, e.g. an energy storage device or a grid that can sink variable power levels. This is a conceptual problem of your project that should be clarified before starting detail engineering.