Which half dollar coin qualifies for Machtzis Hashekel? Can I use a coin that is only 40% silver?

It depends on the weight content of silver, not the percentage.

The amount of silver required (for those that are stringent to use coins that are actual silver) is approx. 9.6g.

The 90% coin has a total of 11.25g of silver, so it’s good. But the 40% coin has only 4.6g.