How you can play Mkds to mk8 (wiiu) online in the current dayMkwii/MKDS (both hosted via wimmfi. can be both played on emulators (melondDS and Dolphin) Note that Dolphin requires a real NAND and you need a computer that can run the game full speed
https://wiimmfi.de/<spacingMK7/MK8 (wiiu) both are currently hosted via Pretendo, a recently made service to revive the Nintendo Network which shut down some months ago.
(MK7 requires a modded console to play online, hack your system at 3ds.hacks.guide)
(Also has CTGP-7, which allows custom tracks to be played online)
(MK8 can be played with an unmodded console via DNS, similar to Wiimmfi, only MK8 is compatible with emulators, MK7 can be played online via CTGP-7 through a Citra fork, but you are put with other Citra players and usually have bots in them)
https://pretendo.network/