The key to success are quick launched rides like the Shuttle Loop which can earn 800$ or more a month easily. For "normal" coasters, use a very long station to increase the guests they can handle and thus, their profit. As for the huge coaster already there, leave it be or not. This ride is a superb money maker, no matter if you run or sell it. However, the ride should not be messed with, as say, for example, shortening the trains to make an other, it might fail going over the first few drops. Simply put a mechanic there, and focus on the rest of the park.