The reason the Corse manual specifies two left hand motors dates back to when the Ford/Mazda motors were a new item to the market. For some reason best known to the manufacturers, the left hand motors had a retail price much cheaper than the right hand ones! When buying new it made more sense to buy two left hands!
These days, with so many MX5's being broken you can buy as many good used pairs as you like so the reason for buying two lefts has gone.
It was perpetuated as another Gooch myth ( so many of those......)
To fit a right hand motor to a left hand bracket is no problem. You either re drill the locating peg hole or cut the peg off all together. Only Corse specific this though.