VNC Server for Mac OS?

There is already VNC-server software builtin to the macOS. Go to System Preferences > Sharing and tick the box for Screen Sharing on the left. Notice the Computer Settings… button on the right. Click that and you will get a drop-down dialogue with an option for "VNC viewers may control screen with password:" Enable that, and you should be able to use whatever VNC-client software you choose. I am not very experienced, but my research led me to choose TigerVNC, which I've used with success on Linux to control my Mac.

The standard app in the old days to control Macs remotely was Timbuktu (Pro).

Selling of the app was stopped in 2015.

• A 68k version is available here: Timbuktu Pro 4.0.6
• A PPC version is available here: Timbuktu Pro 5.2.4 & 6.0
• A macOS version running in Sierra is available here: Timbuktu Pro 8.8.5

To get keys/serials you'd have to buy them at ebay or use non-legal sources.