Best route to take when replacing an old panel without a main breaker
1) can't happen without the power company's consent, and the meter pan and service drop up to the weatherhead must also be rated for 200A.   If those aren't true, you can't do #1. 

2) is out of the question.  The old setup was legal because it was so small it qualified for the Rule of Six (six throws to turn off everything).  And the  maximum theoretical load, all circuits at redline, was 125A on one pole and 150A on the other. That is such a modest oversubscription that since the Rule of Six was allowed, a main breaker wasn't really needed.   However, different deal on a modern 30-42 space panel with 200-400A of breakers provisioned on each pole. Besides, the Rule of Six is now outlawed! You must have a main breaker today. 

That leaves #3, and I recommend switching panel brands to one where a 100A main breaker is readily available.  Unless you plan to have a generator; in that case stay with Siemens but swap it out to a main-lug panel (why pay for the main breaker) and fit their generator interlock.  

You can also backfeed this panel and ignore the main breaker, but I do not like panels where the main shutoff is not obvious.  In an emergency, people don't rise to the occasion, they sink to the level of their training.  In other words people get stupid in key moments.  Someone frantically working in the dark is gonna grab the big handle and go THWOP.  And he will expect the power to be off at that point.  So if that does nothing and you have to shut off one of the regular breakers, well, that's just confusing...