The key obstacle for these levels is the voltage swing.
With 1.2V one can get about 0.6V differential swing across a load. Given the load impedance and the voltage swing, you'll have both the peak power and avg power that you can deliver. It's not possible to deliver ~40mW on a 75ohm with only 0.6V across it, so you'll have to put a balun to reduce the load impedance until you get the power you want, within your max diff swing. It's not useful to go below 10ohms load impedance, in my opinion. Then you can calculate the max current that your output stage will be putting through the load impedance. with that current you can estimate the gm, assuming your diff input swing is also about 0.6V.
There's a bit more that can be estimated, but once I have gm, current and load I'd do some simple simulations to check the frequency response.