Hi,
FWIW - A 34T on your 20" wheel will act like the standard 44T does on a 26" wheel....
Typically the engine with this standard gearing will be revving pretty high at about 30 MPH (I hate winding my dax 70 that far though for any length of time)....About the smallest sprocket that can be custom made for the 9 Hole mount system is 27 T which would be approximately the equivalent of a 35 T sprocket on a 26" wheel......I have a 36T sprocket on my other mountain bike and just installed a speedo on it tonight but it got too dark to ride (no headlight)....so I won't be able to tell you the approximate top speed tonight....I am dying to know what speed my 36 T will pull so I will definatelt test it tomorrow eventing unless it rains.
Andrew