efficient and economic way to UV a duplicated half (-X axis) of an object?

i have a head which is already UVd.
however i had to make some symmetrical changes, hence i deleted one half of the head and made the modeling changes.
now, what is the fastest way i can duplicate and use the UVs from the existing part on the opposite side, so i dont have to do UVs from scratch for the entire object.
hope i was able to explain myself!

thanks in advance for any help

MG