I'd just scale the time in the same way it's scaled now. The important changes would be (a) the cap of city size, and (b) the half food from granary when city loses a pop point. The combination of the two would change the mission from being potentially devestating to inconvenient.
Oh, and in case I didn't mention it - subsequent missions could not raise the unhealthiness above the city's population, they'd just "reset" the mission. So you'd never have more unhealthiness than you had population, even if it was spammed.
Bh
Oh, and in case I didn't mention it - subsequent missions could not raise the unhealthiness above the city's population, they'd just "reset" the mission. So you'd never have more unhealthiness than you had population, even if it was spammed.
Bh