WonderSwan:USB-C Power Mod
There are a few modern power replacement options for the WonderSwan. Since it also takes AA batteries by default, rechargeable AAs are also worth considering.
USB C Mods
WonderSwan Color USB-C Power Module - This provides power over USB C. It does not contain a battery, so the usb c cable must remain plugged in to use it. Quite useful for testing.
Battery Mods
WonderPack - USB C Battery Mod - Battery mod created by FlashMasta (Can be purchased bundled with the FlashMasta and a headphone adapter for a small discount). Contains a 2,000 mAh LiPo, and charges over USB C. Battery life is estimated at 10 hours when using with an IPS modded WonderSwan Color on max brightness. Supports charge and play. This battery replaces the stock AA carriage completely, and sticks out a little bit more.
RetroOnyx WonderSwan Battery Pack - Battery pack by RetroOnyx (the guy behind the E-Ink Virtual Boy Flashcart). Contains a 2,300 mAh lithium cell and also charges over USB C. Battery life is estimated at 11 hours when using with an IPS modded WonderSwan Color on max brightness. Supports charge and play. This battery replaces the stock AA carriage completely, and sticks out quite a bit covering the entire backside of the console. While quite comfortable to use when playing the WonderSwan in horizontal mode, it can be a little uncomfortable in portrait mode.