The Pentax 645N will give you 1/1000th but not interchangeable backs although you can load another insert and swop that at any time. You have to do a mid-roll rewind with the insert that is already there.
While it is a hand-holdable camera it is essentially not a sports/ fast wildlife camera. For most pictures of birds in flight or motorcycle races I'd want the likes of a F100/F5 with a 300mm f2.8.
Two comments: The lack of interchangeable backs may not prove to be the drawback you think they are and with the right lens such as a 200mm or 300mm( about 125mm/190mm equivalent in 35mm) and 1/1000th speed some fast action shots are perfectly possible