The problem with the past is that it's so uncivilized, but any time traveler worth their salt can fix that. Just introduce it to the delights of modern technology, several centuries early. You may need to go through a few intermediate stages, replicating the history of technology on fast forward, but you know exactly what needs doing. How difficult can it be? If a Hero succeeds, there's still a risk of going horribly wrong, going horribly right, or both.