I can't find anything on Vincent powered target boats, but this might be what your Dad was thinking about?
https://www.vincentownersclub.co.uk/...fe-boat.10001/
It's a Vincent powered lifeboat. the thread mentions that there might be one at Flixton, which would be the Norfolk Suffolk Aviation Museum. If so I think I have seen it, but can't remember it having a Vincent engine. It did have "wings" though. More like dart flights really, to make a stable descent from the plane I imagine?
Haven't figured out the engine yet.. Seems to be a three cylinder two stroke with two crankshafts and six pistons. With pistons doing the work of the crankcases for the induction cycle?.. Must read it more carefully.
Maybe it was these engines that powered the target boats?
The Picador's gearbox would drive a propeller shaft though and would not need to be made of Magnesium or even alloy for a boat, so possibly there was a marinised Picador or V-twin? I suspect that the Navy would have gone for the marine engine from the lifeboat though.