Substitute agar-agar for gelatine in no bake cheesecake
Whole recipe would be useful.
Going by various popular vegan no-bake cheesecake recipes,  I see 1 to 3 tsp of agar powder.
Higher ratios of fat will need the higher amount of agar.
Lemon juice as a flavoring in this case shouldn't be an issue. Even half of the liquid being lemon sets up fine as in these bars:
https://thehiddenveggies.com/vegan-lemon-bars/
Personally, I prefer kappa carrageenan over agar in stove top dishes as there is less boil time and partially sets while hot allowing for last minute tweaking.
ie test amount removed to cool surface.