That simple boost converter might be just the thing to convert the voltage--the amperage I don't think you're going to be happy with, though. 3 or 4 amps from a little 9V battery will kill the battery very quickly, even if the boost converter can maintain 50V with that kind of load on it.
What are you planning to use this thing for? If you say what you're trying to accomplish, instead of how you're trying to accomplish it, we can often come up with an alternate solution which may be simpler or work better.
Torben