If you're confident about it, I'd assemble a few details about the run and try and articulate briefly why you think it's cheated. If the mod isn't contactable I'd ask other runners about it, if there genuinely aren't any available either the support hub could well be the way to go, provided you're also a runner of the game. In the support hub you're able to file reports against users for that type of thing but it's imperative to be clear to the site mods who won't be familiar with the game. The thing with cheated runs is it's a very serious accusation to levy against someone, if there's a chance it's a mistake or some exploit you're not aware of, try and rule them out.

Out of interest what game is it?

To be honest, I am not completely sure that it's a cheat so I don't want to get anyone in trouble.

no worries - it actually is a tough one if you're talking about Goosebumps: Attack of the Mutant, only two runners, one of which hasn't logged in in 6 years as you said originally. If it's worth anything though I'd be astonished if someone was committed to faking a run for such a tiny community relatively speaking