I would like to respectfully disagree with siesta181 on the relative costs of 30/45-minute 7734 movements. Over the past year, I have not observed significant differences between the selling price of a 30- and a 45-. Other factors -i.e. condition, watch model- usually play a bigger part in determining the price. It cost me more than $60 for the two necessary parts, and labor may not be cheap, as it involves a tear-down and reassembly of the chronograph components. I believe that $60-100 would be better spent on the correct movement to begin with.
That said, a 30-minute counter will work fine on the Monte Carlo. The only difference is that the counter indicates in increments of 1.5 minutes e.g. {0,1.5,3,...} If a 30-minute movement can be found cheaply enough, and you don't mind the watch looking slightly off on every other minute that the chrono is running, then go for it!
No other movement will fit in this case, unfortunately. It bears repeating that the Poljot 3133 will NOT work, despite appearances.
My avatar is the evil frog archaeologist, Dr. Rene Belloq, arch-nemesis of Indiana Jones. Personally, I think he's just misunderstood...