In my opinion using a modulator IC such as TDA8822T that has a video and sound PLL and will work at any channel and then to up convert this signal up to 2.4GHz and amplify.
But 50W at 2.4GHz amplification is quite difficult.
The channel which will be used for modulator IC will be lowest frequency because of difference minimizing. I mean if modulator works at 45.25MHz vision carrier , Oscllator that will be used for upconverting proccess will be around 2.4GHz+45.25MHz smth.like that.