+1. It doesn't take but a little while to open up a two stroke and clean it up and install new bearings. Bearings are about $15.00. Most of the used engines you buy are going to have some rust in them depending on where {state/area} or how it was put away. If it's free spinning and the bearings roll smooth they may be OK. Rust on the crank is nothing new.
For the price of the new K&B I wouldn't have bothered with a used one. Most the time a tuned pipe is more trouble then it's worth on a stunt plane owned by a sport pilot. They can be a pain.