For the change from THT to SMT…
For advise, it’s important to know that this is a design for an analog transceiver, and it has lots of discrete resistors and transistors that work at frequencies between (I guess) 20MHz and 100MHz. I do not have much experience with this.
I’m also guessing that there is no pressing reason to make the PCB itself smaller. Maybe it even has to fin in the same box, with the mounting holes at the same location.
For the electronics itself it does not matter much whether you put the SMT parts on the top or on the bottom. The most important design rues are probably to keep the GND plane as complete as possible (the original design was not stellar in this regard), and probably also to keep capacitive coupling to the GND plane small. Maybe even remove some parts of the GND plane when lowering capacitance is of bigger concern then the GND plane itself (I’m guessing here)
Capacitive leakage to GND will be smallest the signal part from footprint to footprint is small. Witth footprints placed in such a way that the signal path is straight, and other connections are to the side. Such a routing style would resemble the PCB’s I’ve seen of spectrum analyzers. During the changeover from THT to SMT, there probably is not much of the layout itself you can re-use. The resistors are much smaller, and pins on a SOT-23 are in different order then on a TO92.