I don't mind writing XSL but if one has already been written I should reuse it.
rtept = route point in the GPX format
trkpt = track point in the GPX format
Converting latitude and longitude points from the first to the second, ignoring irrelevant elements.
Unless someone proves me wrong, there is no such XSL application available.
I would have loved to contribute one, but incidentally the whole point of the conversion disappeared when I discovered that GMapToGPX can generate files in the trkpt format directly.