Abu you may still benefit from a hole in the firewall just in line with the carb opening and bigger than the carb hole by about twice the diameter. With the wild air from the engine running it may have problems without a bit of relief in the area. Running it will tell. I think I recall you have run this engine on a stand so if it doesn't perform as well on the model give the hole a try. Gas engines(which this is certainly not) with Walbro carbs (and similar) have a fuel stand off situation and really wont run up to their potential if there is a restriction (wall) near the carb opening. They also benefit from the use of a velocity stack, though I'm not at all sure this engine would, four stroke and with 3.5 cylinders drawing fuel/air each revolution it would have a rather constant flow into the carby.
Doc