Hi

Sorry for this, i told to @arkive to open a new thread for C64 emulation.
I know Recalbox doesn't support Commodore 64 but i think i can answer beacause all i will said will work on Recalbox and others OS.

1st, you can use 320x200 resolution (the C64 max resolution) but it's better to use a 1920 (ou 1600) horizontal resolution for screen setup.
Commodoire 64 have a 200 lines vert(ical resolution but if you want overscan color background, you must open a 240 lines screen.

I use this timing because screen can be horizontaly "zoomed/unzoomed" and moved. Screen can be verticaly moved too.
To "zoom/unzoom" horizontaly, you must setup you CRT to do this.
Due to Rpi Pixel clock limit, if you use a 320x240, you can't do anything (zoom/position)

50Hz PAL
vcgencmd hdmi_timings 1920 1 64 192 224 282 1 8 5 18 0 0 0 50.0 0 37560000 1
60Hz NTSC
vcgencmd hdmi_timings 1920 1 24 192 184 240 1 4 5 14 0 0 0 60.0 0 36609600 1

VICE binary and configfile are in :
/recalbox/share/emulators/combian64/
ALL other VICE files are in :
/recalbox/share/emulators/combian64/vice/
(it's because i use combian C64 version and i choosen the parent folders)

And for config file :

*** NTSC ***

[C64] Directory="/recalbox/share/emulators/combian64/vice/" MenuKey=293 MenuKeyUp=273 MenuKeyDown=274 MenuKeyLeft=276 MenuKeyRight=275 RefreshRate=1 SoundDeviceName="alsa" SoundBufferSize=100 SoundSpeedAdjustment=1 KeymapUserSymFile="/recalbox/share/emulators/combian64/vice/sdl_pos.vkm" KeymapIndex=2 KeyboardMapping=1 AspectRatio="1.500000" SDLLimitMode=1 SDLCustomWidth=1920 SDLCustomHeight=240 SDLWindowWidth=1920 SDLWindowHeight=240 SDLGLAspectMode=0 ChargenName="/recalbox/share/emulators/combian64/vice/chargen" KernalName="/recalbox/share/emulators/combian64/vice/kernal" BasicName="/recalbox/share/emulators/combian64/vice/basic" MachineVideoStandard=2 VICIIVideoCache=0 VICIIDoubleScan=0 VICIIDoubleSize=0 VICIIFilter=0 VICIIBorderMode=1 SidEngine=0 SidModel=1 JoyDevice2=4 HotkeyFile="/recalbox/share/emulators/combian64/vice/sdl-joymap-C64.vkm" JoyMapFile="/recalbox/share/emulators/combian64/vice/sdl-joymap-C64.vjm" Drive8Type=1541 DosName1541="/recalbox/share/emulators/combian64/vice/dos1541" Acia1Base=56832

FOR PAL, you just have to delete this line:
MachineVideoStandard=2
And use 288 lines :
SDLCustomWidth=1920
SDLCustomHeight=288
SDLWindowWidth=1920
SDLWindowHeight=288

The command line :
/recalbox/share/emulators/combian64/x64 -config /recalbox/share/emulators/combian64/config.cfg -autostart "/recalbox/share/roms/c64/Bubble Bobble.d64"

That's all

No support here.
If you can't launch C64, VICE must be correctly installed.

Screens from NTSC :
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A 43 years old guy 😉