Just found this very old post so thought it would be worth digging up, top speed always being an interesting topic.
The current record is held by a KX500, google "card board robot KX500" to see the current record holder. Unfortunately, I think really goes against the spirit of what this is about.
Knowing the original record was done on an otherwise stock bike, (stock plastics, suspension even tyres) the KX500 has gone way too far.
Not only has the thing been radically lowered, it has skinny road tyres and most of it's bodywork removed, a dirt bike? I don't think so.
I recently emailed Maico International with interest about doing a record attempt, as after doing quite a bit of research I figured a new Maico 700 (the special model soon to be released) with a claimed 100HP would be an awesome base for such a record.
Unfortunately for me, they already have someone in house who is aiming for the same record, so it looks as if their support is going that way.
Will be interesting to see how they go with the record, the new 700 should be able to smash through it while looking very much like a dirtbike.