NOTE: THIS TUTORIAL ONLY WORKS FOR BUILDING THE x64 VERSION OF THE DECOMP!

Firstly, you will need to install the Community version of Visual Studio 2019 (which is free, found here)
Next, you need to download all of the dependencies needed for a successful build of the Sonic 1/Sonic 2 Classic Decompilations, which can be found here.
Next, go to the location where you downloaded the beta build of the Sonic 1/2 Decomp. There should be a "dependencies" folder in there. Replace that folder with the dependencies folder you downloaded from the second step.
Next, you need to load up the file "Sonic12Decomp.sln". (The Visual Studio project file)
Press "F7" to build it (or any other way of building it works too)
There you go! If done right, you should have the beta build of the decomps, well, built! The built files will be in the newly-made directory "builds\x64\" (I believe there is another folder after that, you may have to do some fishing around in there to find it)
You should find that there are 2 files you may recognise: RSDKv4_64.exe and SDL2.dll.
Copy those and go to the location where you have the official release build of the Sonic 1/2 decomps, and replace the files.
Run the RSDKv4_64.exe file, and you should be good to go!
Note: When attempting to run the file, you may run into an error saying you need a file named "vcruntime140.dll". The file is already included in the dependencies folder you downloaded and should be in the root. Place it in the same directory as the RSDKv4_64.exe file.

TL:DR
Download Visual Studio 2019 (here)
Download the dependencies folder (here)
Replace the dependencies folder in the source code of the decomp with the one above
Run the file "Sonic12Decomp.sln"
Go to the folder "\builds\x64" and grab the RSDKv4_64.exe and SDL2.dll files
Replace the original files in your 1.1.0 version of the decomp with the newly built one
(if needed) Move the "vcruntime140.dll" file from the dependencies folder to the same location of the new RSDKv4_64.exe file

GET THE MENU ASSETS, PLEASE!
Place the Menu folder in the same directory as the Data and Bytecode folders, and it'll work.
https://drive.google.com/drive/folders/1YhrynkayllKulmsR3VhswT2LLc7u6VHC