Wattage is important also, so I'd parallel the two 16 ohm resistors to mimic the MTH smoke unit. The small smoke unit from MTH has one 8 ohm resistor, and it requires different firmware for PS/2 (or for PS/3 a different chain file). That being the case, a single 8 ohm resistor will likely not give you the desired effect.
For best results here, mimic the PS/2 stock smoke unit.