Go here:
Perry Pumps - Carbs
This is a list of the carbs they have available. Scroll down until you get to ST and then the .90, pick its part number and go from there for ordering. I would suggest you call them just to confirm what you found on the website and other logistics if needed. Your all set.
If you go the Perry route, and you need to adjust the mixture, make sure you follow their directions, as when adjusting the mixture disc (it has an inscribed '+' or '-' on the side with a slot cut in between, IF needed, move the disc ever so slightly, like a 1/64 of an inch, (not a misprint) that how sensitive it is to set, once your happy with the engines performance, most likely you will never have to set it again.