From what I read, the GWR bogies and what is now the outer track were replaced in the 1968-69 upgrade. An inner track with 4 additional bogies (don't know what type) to take some of the weight was added at the same time. A blog post by Didcot Railway Centre says they did look at the wheelsets out of the GWR bogies some time after they were replaced, but were found to be too badly damaged.
While we're on the subject, the Australia Telescope Compact Array sits on a 3.2 km long track made from Australian Standard 68kg/m rail to a gauge of 9.6m. The Australasian Railway Association officially recognised it as a "railway" which would make it the widest gauge in the world.
The complex which includes the ATCA is named the Paul Wild Observatory. Astronomer Paul Wild was a lifelong railway enthusiast and as child in the UK wanted to be a train driver specifically to drive a GWR King class.