Simon Williams' "costume" for his Wonder Man transformation is based on the original costume he wears during his debut in the Avengers comics, as a member of the Masters of Evil.
In this series, Eric Williams is already the Grim Reaper before his brother Simon Williams gets his Wonder Man powers which is a reverse from how the events happened in the original story from the comics. In the comics, Zemo was the one who gave Simon the Wonder Man powers and Eric became the Grim Reaper to avenge Simon after his first "death".