Tuned active mixer have about 20dB gain, but it sometime oscillate by itself and makes unwanted signal. Wideband passive mixer have no gain and no loss, and it works stable. So I like this type of mixer. Mixer makes not only 50MHz USB signal but also 70MHz LSB signals. So you must stay 50MHz band pass filter after the mixer.
Power amp is constructed by wide band amplifiers.
I will not describe about it, because it is written in many books. But I say you that to avoid the self oscillation, you must care about some points. At the end of power amp, the low pass filter is located. I will explain about receiving time. This pair of front-end circuit is very popular as standard for amateur radio builder in Japan.