MOT emmisions aren't the same ones used for Tax purposes.
Tax emmisions are calculated by the manufacturer and declared, they can't be done on a car that's had an engine change.
I'd guess if you use a post 2001 low tax car and put a post 2001 engine in the Tax will revert to pre 2001 on engine size
The MOT emmisions are definitely engine or body year related, whichever is the oldest.
So 2005 engine in 1998 body = 1998 emmisions.
My year 2000 Focus has a pre 1993 engine in, so will be MOT'd at pre 1993 emmisions, when I eventually get it on the road...


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