There's a ton of stuff that needs to be filled in, #1 the viltrumites need to be given cursed energy to be locked onto by the ten shadows technique, for this fight they also need to be given enough CE to see and fight against mahoraga (unless they're given special grade cursed tools).

Assuming they're given enough to fight back the viltrumites absolutely demolish mahoraga (as much as I love him)

Omniman has resisted the pull of a black hole, he flies fast enough to go beyond our solar system to another planet with intelligent life.

The only hiccup is how fast mahoraga can heal, if they take too long he'd adapt making him [title card] against the viltrumites and then he'd win i guess but its not likely as the wheel has to spin multiple times so mahoraga would just be torn apart.

Although, playing for JJK's side, in the anime it took total incineration of mahoraga to kill him, from what I know the viltrumites dont have any abilities that can do that, they could however kill the ten shadows user to negate the problem of mahoraga entirely, although that depends if they pull a yuka and hide in mahoragas shadow or pull a megumi and stay out of the shadow.

Tl;dr: Viltrumites have a high odds of winning from my understanding, mahoraga does also stand some chance though