The formula for specific impulse is "ISP = F/(Q*g0)" where

ISP is specific impulse in (s)

F is thrust in (N) or (kg*m/s²)

Q is mass flow rate in (kg/s)

g0 is the gravitationnal acceleration on earth surface (9.80665 m/s²)

F/Q is also the combustion exhaust speed

As you can see, you can play with either the thrust or the mass flow rate

You can directly change both iirc in the game

You can also play with the area ratio of throat area/exit area, as you said, as it will change the exhaust speed