Once upon a time I wrote scripts in C #, making my game. And when I found out that you can use C # scripts on unity, I thought about this issue. All the same, the game turned out to be normal, but physics and optimization suffered. That’s why I decided to try unity for this!
Plain old C# can be copied into a Unity project and will compile and run normally.
If you want to take advantage of Unity’s systems for physics, rendering, etc. then whatever parts of your old code handled that stuff will need to be rewritten or replaced with something Unity-specific.