@red732 a shader could do this. eg: https://github.com/libretro/glsl-shaders/blob/master/misc/flip-horizontal.glsl

any arcade game that was available in such a cabinet probably has a system option to run it in a flipped mode - maybe in the service menu or dip switches. eg https://retropie.org.uk/docs/lr-mame2003/#service-menu

@phrazelle you can fix the ES L/R by editing the es_input.cfg file.

go to /opt/retropie/configs/all/emulationstation/ open es_input.cfg and switch the values for right and left. This will only effect the controller for Emulation Station and not effect the emulators.

Better option is to bring up the retroarch menu (select + top button) then go to "Quick Menu > Options" and set "Invert Y Axis" to "Enabled".

Not sure if this helps, but this link will take you to a script that will install the xwiimote and xwiimote-tools packages.

Once installed you should be able to connect and configure the wiimote using the instructions here.

Hope this helps!