Hoi Rick
I don't think it's a good idea to install the Ryzen 7. The chipset of the Asrock A300 comes entirely without a graphics unit. This means that the processor has to take over this task. Therefore, you can only use processors that contain a graphics unit, such as AMD Athlon 3000G, AMD Ryzen 3 3200G or AMD Ryzen 5 3400G. (Only AMD processors with "G" or "GE" at the end of the name). So in my opinion this will not work with the AMD Ryzen 7 2700.

If you still want to take a chance, there are several challenges: It will be difficult to cool the processor inside the device, because it consumes max. 65 W TDP and gets correspondingly hot. So you would have to install a cooling system that goes beyond the mass of the case. In addition, you would need a graphics card to be able to display an image at all.
Because the Deskmini does not have an internal graphics card, you would have to build it externally. Start a Google search for "asrock deskmini a300 external gpu" and look at the solutions there.

In the end, you have to judge how much money you are spending on such a solution and what performance you are effectively getting for it. Maybe it will turn out that a larger housing is simply more practical.

For information: I have two Asrock A300s in use. One with an Athlon 200GE and one with a Ryzen 5 3400G. The weaker version is COMPLETELY SILENT thanks to the Noctua cooler and is suitable for surfing, office work and streaming/video projection without any problems. The variant with the more powerful processor is also completely silent when idle + under low load, only during somewhat more computing-intensive tasks (e.g. image editing) does the fan turn a little faster and comes close to the lower limit of the audible range. (I don't do any gaming...)

Your CPU needs an onboard graphics card.
Here is what I have put together:
https://www.digitec.ch/de...
(data without guarantee)

Here is the compatibility list:
https://www.asrock.com/nettop...
Regarding your question: Ryzen 7 not listed. Even if the socket is correct.