I found many interesting gadgets on dev-c.com/gtasa/, such as "No Begin Screen Patch", "All Cars Radio&Repair Patch", and "Menu Editor".
What they have in common is that they can directly modify the binary code in gta_sa.exe. Unfortunately, the "No Begin Screen Patch" and "Menu Editor" of them only apply to the 1.0 version gta_sa.exe, not the 1.01 version. So I decided to do it myself. I removed startup screens and added a “save game” button to the pause menu in version 1.01.
In addition, if you want all cars are allowed for a respray and FM-radio, you can use "All Cars Radio&Repair Patch" to modify the 1.01 executive file. This patcher is universal for multiple versions. If you don't know the difference between version 1.0 and version 1.01, just google it.