John,
A size limit, or configuration rule would be the easiest way to limit the
speeds. As far as how much speed can a 12 year old handle?. That really
depends on the kid. Some are going to be able to handle more than others due
to natural talent and lack of fear (common sense?). Here is where a
graduated or progressive type of license like adults use might make sense.
Just, a weird thought, how about a Briggs & Stratton or Tecumseh type
engines with various type of restrictor plates, based on a license. Go as
fast as you can, as long as you use a certain size restrictor. It would work
with any carburetor engine.
Dale "Chief" Thomas
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