10:2 reduction gives you 350 rpm, or 5.8 cycles per second. That's still pretty fast to [try to] move something as big as your shaker frame, unless you're only moving it a small fraction of an inch (like you jigsaw did). I have no experience with soil sifters, but I suspect you need to be going something under 60 rpm at the final crank pulley. The pulley broke because the spokes aren't made to take loads in that direction. You probably need the motor with its 2" pulley driving a 10" pulley on an intermediate shaft, then another 2" pulley on that shaft driving the existing 10" pulley on the existing shaft. On that shaft, make a crank arm (you could modify a steel, not cast iron, pulley or gear, or make it from scratch) to connect to the connecting rod. The crank is your existing 10" pulley. The crank pin is where you attach the connecting rod.