If I were in your position, I would probably try to take the primer and ideas and history of colonizing the United States and work more aspects from history in since that would be lacking in magic. I'm not familiar with the Iron Heroes rules, so I can't really offer a lot of mechanical advice, but another idea is perhaps making the "new world" much more hostile with more monsters and harsher conditions thereby extending the phase in which the characters fight to gain a foothold...