Yes, the screw is missing. The result is that the carbs will close completely and timing will retard to the point where the engine will not idle or run. It may also cause a no-start condition.
How can a mechanic miss it? He probably was not familiar with Force engines, was in a hurry, or was just plain sloppy or incompetent. I can repair any Force engine with my eyes closed and have done so many that a simple thing like that just pops out as wrong. However, when you see a tapped hole with no paint on it and scuff marks , it should tell you (if you are good) that something belonged in there. So we will take away marks from him for that oversight.
You need a stainless 1/4 X 20 fully threaded screw about 2 inches long and a 1/4 X 20 nut. The nut goes on the outside to lock the screw and keep it from backing out. If you can not buy one at Sears, Lowes, or Home Depot, try Fastenal.
It doesn't appear that the hole is stripped but if it is, you can use a nut on each side to locate the screw OR you can tap out to 5/16 and use a 5/16 X 18 screw and nut.
Put the engine in forward gear (to make it easier to thread the screw in) . Thread the screw in until a little more than 1/2 inch is showing at the back. Put the engine in neutral and see where the scribed lines on the throttle cam are located. They should be slightly below tangent to the carb roller. Watch the video, It shows approximate position at idle or neutral. Start the engine, let it warm up, and adjust the screw to 700-750 RPM in forward, in the water. If you don't have a tachometer, adjust to a reliable idle in forward gear, in the water. The engine should clunk when put into gear. It should be a solid sound but not too loud and not crashing or grinding. Remember, when putting into gear move the control lever quickly. Do not try to "ease" it into gear.