Most of us are using tractor pulleys, those cast iron ones... but yes, a lot of us use lawnmower pulleys as well. It really depends on how much power you're dealing with and a stock 2 stroke china girl really don't make a lot of torque that can break stuff, but a heavily modified one is a different story.
Which engine are you using or planning to use? and will it be modified to make more power? Depending on the size of the pulley, and what part of the drivetrain it's going to be used on along with how much power we're talking about plays a big role in how strong it really needs to be. A small diameter pulley on the crank would be able to endure a lot more than a larger diameter pulley on the wheel where the torque at the center will be considerably more etc..
Also, do you have a pic of the pulley in question? I may be able to tell if it can hold up by seeing it and knowing what size it is.