Mine was built by Chris at MWH. It has the Polini kit on it, race crank and bearings and Proma circuit. 15mm BING. Geared at 18/40 it was weak on pulling until about 20mph, then it ran and ran. I'm 220 pounds, though and the Magnum is heavy. I'm sure it could go faster, but I was running it 1 jet too rich to prevent blowing it up. Mitch Farley can tell you about how it ran.
The 2 speed is slightly stronger on pulling than the 1 speed when stock. It's not nearly as reliable and harder to make parts fit to if it has oil injection. Best left as stock.