Yeah there's some things called "universal wifi adapters" that are just basically a one port WAP, connect to cat5 and NIC... then you can use most of those "Wifi plug"/range extenders that are a wallwart with antenna and a network jack in that mode too with a bit of futzing with setup. There are also ISA and PCI PCMCIA adapters and probably prism2 drivers for older OS that you can use a prism2 PCMCIA card in. Then since you ain't gonna get much speed out of a retro box, you can also pick up discarded Powerline adapters that are on the low speed end, and while it might take XP up to configure them to connedt, one by your retro box, one by your wifi router, hooked up with cable, then when they join and network to each other you can use anything on them and it's transparent like they're on a network cable. Then also there are options using ESP8266 and ESP32 wifi dev boards that hook up to serial and parallel ports and may emulate a SLIP, PPP or PLIP connection. Also, just for getting shit networked somehow, you can use a 4 wire laplink parallel cable with Windows Internet Connection sharing on up to Windows 7 I think and there's a dos packet driver for that.
Edit: if you can boot to win98 and use a USB card/ports you can get USB wifi adapters (old ones) working too.
EditII: Universal wifi adapter is getting bad search results, the artificial stupidity is outsmarting itself, ethernet to wifi, or ethernet wifi bridge, seem to get better hits.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.