Well the mot test manual has been changed from draft to in force from today and has some interesting changes on the emissions front for Kit Cars.
If a kitcar was first used before august 1998 it is treated differently to a car with first use after 1998.
If its the latter its tested to the emission limit shown on the v5c, if the v5c has no emission limits showing it will be tested to emission standards as of its actual date of first use.
So the way I read it if you had a kit that was first used say 2015, it would have to pass emission limits as of 2015 regardless of engine used, if there is no emission limit value on the v5
The old approach of using an older engine and testing to engine age only appears to now be valid on a vehicle first used before 2002.
It appears pre 1998 kit cars are not affected, they are treated as first used before august 1975 and don’t need emission testing, unless I have read it wrong.
Manual is available here