Depending on the condition of the carbs that are included with the engine from this guy, you may end up putting the same amount of time into getting them cleaned, jetted, etc...that you would spend just fixing yours...
I am heading back down the the shop in a second...I'm almost certain that I have a spare set of 500/550 carbs that I used for parts, and will see if I have the mounting plate and leaf spring that you need and will send em your way for the cost of shipping. Then you can just buy the gaskets or other bits and get yours running for a fraction of the cost.