My understanding is that PR-2 is in a dismantled state in the museum store and had donated an engine to the Gauntlet?
Post#58 here, http://forum.keypublishing.co.uk/showthread.php?t=33512&page=2 seems to go with that as well,
PR-1 was written off after hitting a snow bank in 1965, but maybe componants were kept from it.

May be also from PR-2, but I would have expected the engines of PR-1 having been available for the Gauntlet, as they were not damaged in the incident. Not sure though…