A 2 stroke twin, and not the view you'd usually see of it !
Likely candidates that come to mind are a Villiers 2T twin, the Excelsior Talisman twin, or an Ariel Arrow.
Views of the gearbox end of it and the timing side could assist.
The Villiers was the only one (?) that was semi unit (bolt up gearbox), so I'd opt for that one.
Not being real familiar with any of them...
If you can get the same orientation, this would be similar (?)
Obviously, you only have the inner crankcase bits...