It is wholly dependent on the specific charges and how the GOP reacts to them. If the GOP distances itself from Grimm and calls on him to resign, then his odds of winning are close to 0%. If the GOP supports Grimm and helps him fight the charges, and turns the issue into one of selective prosecution or political persecution, and Grimm runs commercials depicting his work on behalf of victims of Hurricane Sandy, then I think that his odds would be about 50%. I don’t think that voters in Staten Island and Brooklyn will get all riled up about a restaurant owner who paid some employees in cash.
They just disinvited him from a fundraiser but I don’t think there have been any calls to resign.