You have two solid options. You could convert him from the Witch Hunter Inquisitor (with hat) or from the Mordheim Johan the Knife fig. The
WH has the right hat, and more
40K accessories, but Johan has the cloak and the twin knives plus back-ups. In his case you'd probably have to putty the hat and mask. Either way you'd be close.
Johan is still available, as the
Inquisitor, even though he only comes in the box set. There was also, I belive, a special release
WH inquisitor who has a similar hat, alhtough I've no idea about his availablity.