Could you make a step by step tutorial I'd really appreciate it.
once you have the zipmod, open it in winrar/7zip (whatever unpacking software you have)
You will see a file called "manifest.xml"
and then you extract and then open that file (in notepad) and edit the line where it says <game>Koikatsu Sunshine</game> and remove the word "sunshine" or add a new line <game>Koikatsu</game>
And after editing and saving the file, add it back into the zipmod and should be all good to go.
hopefully the screenshots make things a bit clearer and most mods for sunshine can be added to OG Koikatsu this way, as long as the mod is purely cosmetic since they both have the same naming convention and structure (as they are both unity).
you can also make the bellies into accessories so you can adjust the sizes and placements too and work around the outfit clipping but that is a bit more complicated (export the body model (.obj) and follow KoikDaisy tutorial to make it into an accessory (will need access to Blender or something similar to remove the T-Pose body from the model).
I personally recommend sticking with Koikatsu/Party as there are more mods and nothing really changes (CharaStudio and character design wise).