yes mate. I understood that. and why I went ahead and got cheap parts to experiment with. that left hand brush set over here in the UK cost about 10bucks the starter complete 15 bucks. but like you I had no intention of scrapping a perfectly working brush set and starter to do it.
by the way FYI that brush set would not rotate enough as the positive brush is fixed to the mounting plate that goes through the end cap. the cheap ones have a long braid to the connector that is loose and not fixed to the brush set. like this one.