Yep, use oakley A frames. Spare lenses are cheap. But you want to have the visor back for road riding, as its a bit breezy.
If you're doing a lot of offroad, Id recommend a cheaper, lighter MX helmet and save your lovely TourX for posing on the road.
I use Oakley L-frames, fit nicely over specs, but the A-frames are fine for non-spec wearers. Handy of a summer evening, flip up the dark visor and slip on the goggles at dusk