As you can see from the photo above, they can convert other McLaren's to look very similar to an F1 LM, but there will still be several differences. Maybe not ones that the average viewer will spot, but those of us with a keen eye for detail and a passion for McLarens would never be fooled.
I highly doubt they would ever badge a converted car as an LM, or modify the chassis plaque, so you would always be able to tell in those ways.
>8^)
ER