Mine has roller bearings on the crank (stock) and turns a 18x6 zinger prop at about 7500 RPM and I am working on getting some more RPM out of it!
It is a single ring engine, modified a bit but a stocker will turn a 16X8 prop just as fast I have tried it.
I took one of mine out of an old worn out trimmer, and the other was new but both run fine.

Have fun with the project and drop me a line if you have any questions.