Well typically you're going to port and take the cylinder to its limits. Effectively destorying the overall cycle life of said cylinder. One thing I learned in Engineering school is that if its not designed or specifically built for the application, its probably not going to last long...thats why they do computer generated life cycle testing.
Here in KC some of the people are having a winter 50cc stock clyinder buildoff. I dont think the cylinders are being built to ride more than a couplel of times. If you push the limits then you hit the limits.