Well it seems that the naturally aspirated engines have reached the end of their life because of excessive CO2 emmisions. At this point there is absolutely no sense in re-engineering the trad chassis to take small turbocharged engines.
I know nothing but I would have a little bet that 3 and 4 cylinder engines are already in CX series prototypes in the "skunkworks" at the Morgan factory, I would almost bet that they will be ready for release at the next Geneva show. The market for CX Morgans with a more "friendly" power plant will be much greater than that of the Plus Six. Due to volume production of the tub and lower hours for assembly I would also expect prices that will be comparable with current 4/4 and Plus 4 prices.
Perhaps even lower

I think there's a place for a reasonable spec Morgan under £40K but seeing the first edition Plus 6's with the CX chassis are £92,000 upwards I also agree with the flying pigs!
There are still some unsold 12 month old 'new' unregistered 4/4's for sale nationally and one of the reasons for them remaining unsold perhaps is the 2017 onwards £320 annual road tax surcharge for the first 6 years on cars costing over £40,000 new
As much as I like my 4/4, I would find it hard to justify £160 plus £320 ie nearly £500 per year road tax for 1.6 litre weekend car, when my everyday cars cost less than £200 a year for both of them.