It all depends...
To me, it depends on the machine. If it's something super rare or historically significant, then I'd come down on the side of being a purist, but if certain parts are simply unavailable, then the best thing one can do is use the closest substitute. On the other hand if it's something fairly common, then why not have some fun and make something you want? I remember a few years ago seeing a thread on here with photos of some really cool custom Kirbys done by Chicago Mike.
I have 'customized' a few of my vacuums, but generally speaking, what I've done is reversible. For instance, I hate dump bags, so I've retrofitted every vacuum I own that came from the factory that way with a disposable bag system, but they could just as easily be put back to their original configuration. I think the most radical modification I've done is a repaint of the shell of an Electrolux PN4A. It's intentionally different from what they did at the factory but It still looks cool.