DES MOINES — The voting rights of roughly 35,000 felons in the state who completed their sentences were restored this summer by an executive order, but only 2,550 — roughly 7 percent — of them had registered to vote by August in this fall’s election, according to Iowa Secretary of State data.For advocates who have been pressing for the automatic restoration of voting rights for Iowa felons who complete their sentences, that number represents a good start — but shows more work remains.“On the one hand, it’s great that over 2,500 people have become eligible to vote and have exercised that right by registering,” said Mark Stringer, executive director of the Iowa chapter of the American Civil Liberties Union.