We have been speculating on the +4, and other than the more powerful engine and the narrower wings TMers were good at the forecast.
Now for the 4/4 model.
The BMW 1500 3 cylinder is a brilliant engine but no easy off the shelf in line gearbox.
The lesser powered 2.0 litre BMW would be easiest but to

o close to the Plus4?
Ford 1600 ecoboost available in various guises and will mate to the MX5 gearbox. Ford crate engines are also really cheap giving Morgan room with the price point.
It would also add to the long history of the 4/4 for special editions like the 150th!!