My 1020 manual shows the timing marks are aligned with a special tool, essentially a straight edge. With the straight edge aligned between the centerlines of the crank and camshaft gear, the mark on the cam gear should fall right underneath the straight edge. Same with the injector pump gear if you have a diesel. Presumably this is done with the crank on top dead center for #1 cylinder.
If this is not clear, ask your question in the John Deere Ag tractor subforum. Any you might put 1020 in the subject line - you'll likely get plenty of responses.
Jim