You will need an engineers certificate even for a pre 68 car if you decide to fit an engine that requires modifications to fit unless it has been done many times before and is conducted in he same manner (so basiclly any engine convertion in a mini ). Also if the engine being fitted comes from a car that is 1974 or newer it must retain all emission equipment. (factory ECU, charcoal canaster, EGR, cat. converter and whatelse may be fitted) Otherwise it must pass an emission test, and they can do them a Regency now at $500 a go.
My engineers bill was around $1500 more mods will cost more

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4efte powered Clubman
supercharged 998 race car

My quote/guesstimate price for the report + testing etc is around $2500, not cheap but an expected cost.

In other words, stick A-series unless you really want to do the project.

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1964 Morris 850, 1330 Supercharged - 81.8hp atws.
1975 Leyland Mini S 1100S powered - Nice and reliable.
1977 Leyland Mini LS - Project LS-T