Reason they're rated at 50mph is they are really small in terms of contact on the road, so they effect braking and stability hence the lower speed limit. Also, you're only ever supposed to use them on the rear anyway, if a front goes you're to put a rear wheel on the front and use the space spacer on the rear.
To be honest, if you're replacing the space saver, ditch it and get a full sized spare wheel instead... That's what I would do anyway.