You can get red / orange lens repair tape from halfords to repair a cracked lens, as long as it shines red light then it will be fine. I very much doubt it will fail because there is no reflector on the o/s, but if it does, then blue-tack a reflector off a bicycle on there. It's what i did to get an old car of mine through the mot when i had different rear lights on it with no reflectors, and of course, it 'fell off' on the way home!