With them 2 strokes, engine lockup is normally caused by the chain binding suddenly in the output sprocket housing. A jackshaft will not fix this, but fixes exist, use the forum search function. I like the one where you fill in all extra space there with epoxy. Its normally caused by a loose chain or a sprocket that has an elliptical rotation. Its better to fix the cause than to do fixes to prevent the symptoms.A larger head would look better in my opinion.
Does a Jackshaft prevent engine lockups?