yeah stock, these things weren't that great on top-end/top speed...you could have it ported/case matched for top-end, and have the head re-worked...but...you'll lose some bottom-end.
I threw on a complete PC exhaust...gently massaged the cylinder, by just smoothing out the casting flaws/rough edges...I'm running a Wossner piston/rod...and found running my gearing at 12/47 gave the best top end over run. I also played with the jetting for quite some time, leaning it out slowly, and I'm running 25% pump fuel and 75% C12 at 40:1.