There is the arguement that how original is the car anyway, they can end up like 'Triggers broom' once you've fitted a new engine, panels, and had the interior re-upholstered is it the same car that rolled off Ford's production line. Also, as mentioned some of the parts are impossible to get hold of. And why would'nt you want to uprate the brakes with something modern that works.
When restoring my Capri I have used bolt on mods but all these can be reversed and the car looks as Ford intended from the outside. My Cortina was in a pretty sorry state and I wanted to make a trackday car so I have gone with a race theme but it could all be reversed with a bit of angle grinding. However, if one of my cars was a special eg. an RS3100 Capri I would certainly keep it original.
I can see both sides to this debate, I think it's your car, your money and it's you that has to live with the car at the end of the day. I'm sure in the height of the 70's custom scene people didn't think twice about cutting up an original car to jack it up, paint it purple and cover the seats in leopard print faux fur.