I just reviewed my references and the 70 and 71 used the 3.25 tire, after that most years use a 3.50 x 19 front tire.
Most tires now use a metric number. A 100/90-19 is a good size. A 110/90-19 works too.
On the rear a 110/90-18, 120/90-18 maybe, depends on the tire. Some 120's fit some don't.
If yours is new enough for a 16 inch rear then a 130/90-16 is the right size.
Leo