I just done this, or sort of. My late Spit indicator stalk broke. Right hand stalks are about £60 and left hand about £30 from Ang Triumph.
I bought a left hand and moved it to the old right hand bracket. Not easy as when removing bracket some springs may drop out and the indicator ball bearing.
You will need the lefthand bracket to do this. I did thinks of putting it on the left but the wiper stalk would be back to front and didn't want to buy another. Plus I wanted to keep the quirkiness of the indicator on the right as all British cars once were.
Danny
Ps if you want the left hand back plate, your welcome to it.