On many full size engines they were able to do without the slots or extra links since the valve travel was very short in relation to the length of the crank arm, so the slight slop in the gland packing let the valve rod take up the tiny offset movements. I've done the same on a few models, they work just fine with no slots on the arm. Worth doing the math on your design to see if you need the slots, if the offset movement is only a couple thou you can skip them. If the crank travel is very large, I'd  go with the link since the slot would bind slightly  when at the ends of the travel.