Hip flexors are very rarely a limiting factor in sprinting. I’d focus much more on posterior chain.
A2G squats do great things for your hip flexors – while really working your glutes, hams, and quads.
Hip snatches and properly executed (with hip extension vs. using your back) hang cleans help as well.
Above exercises are way more bang for your buck than hip exercises, although all of Mattyocs suggestions are great things for including in your abdominal circuits.