I think the problem you're hitting is that the pedal/wire doesn't move the throttle lever far enough to hit full throttle. You can try to adjust it at full throttle like mentioned above, but would be concerned that it may not return to idle like that. Be sure to securely block it up or remove the chain before trying.
If it doesn't work out, you'd probably be best off to rig up the wire to the throttle plate like most do. I did it homemade, but here's something I found which shows setting up a clone throttle. Sounds like they may even sell a kit...
http://www.mxk.com/engines/lifanlink.htm
BTW, great job! I think that the bearings are fine as-is. If you'd ever want to put a monster engine in, then add some bearings near the wheels. This should be okay for 6.5hp.
Enjoy it!