Simple, Let's imagine a Touhou game like titled "Number One Under and Above Heavens" where Goku is the final or secret boss. Even if he uses the strongest version of Ultra Instinct where he materializes a giant plasma version of himself, and that is his strongest form, Reimu will still win, as she always win against the strongest opponents in the games lol.
Of course I'm just trying to be somehow creative, because my standard answer for any crossover battles is like what erlking said, wins who the battle's writer wants to win or who is the favorite character (and that will constantly count for non-crossover battles as well). Trying to be "realistic in-universe" Goku's speed and strength could be way too much for Reimu to handle, but hey every new antagonist Goku faces is physically stronger and faster than him, also superior to his latest transformation unless he trained, so try imagining one of these new antagonists being Reimu, she could be equivalent to Gas or Gomah (the latest two villains Goku faced)
Countless possibilities, the easiest to do imo is applying the rules of one universe only, instead of keeping the other character completely intact, so Goku in the context of the Touhou universe and its rules would lose to Reimu I think, as she being one who solves incidents, she always faces anomalies and figures out ways to deal with them, so she would eventually find her way to deal with Goku's UI. There's a lot of gamification in this too of course after all it's comparing a character from a comic series where the stories has its rules that can lead to the protagonist losing, with a character from a videogame series who basically will always win because the one who's controlling the protagonist is the player and as the player you have to win. Because of this maybe the best comparison would be between Reimu and The Goku from most of the old Z movies where he magically has or finds the power to defeat the villains because those movies follow the rule of him always being the winner XD
I’d say Goku probably wins in a straight-up fight, mainly because the difference in raw strength, speed, durability, and combat experience is huge. Reimu does have some unusual abilities that could make the matchup interesting, especially her ability to float and use barriers, but it’s hard to see her keeping up with Goku physically. That said, cross-series fights get messy when you start comparing how each universe’s abilities work, so there’s definitely room for debate.
Also there are a few cases in the series where opponents weaker than Goku were able to give him trouble because they had unique abilities. I don't know much about Reimu's powers but she could very well be one of these cases if physical strength wasn't the solution for this, though Goku also learned a lot of extra physical abilities as the series progressed, but Reimu's could be more advanced as such.
