Hey,
Well my friend just bought a car and it has a engine conversion because the old engine went bad on the previous owner and now it has a toyota Instead of a Mitsubishi, but its a petrol to petrol conversion and when he bought it he has asked the owner if there is a problem with out this engine being registered and if he will have problems in the future and he has told him that a petrol to petrol Engine conversion isn't sensory to be registered and in fact the owner has the license renewed to 2013 and the eco test completed with the engine not registered and this vehicle is a 7 Sri.
Hope I explained it clear hoping for a reply soon.
Cheers
PS- And We asked some more people and they say that it isn't necessary and the eco test isn't necessary if its under 10Sri ?