It's in 14 CFR 1.1:
"Light-sport aircraft means an aircraft, other than a
helicopter or powered-lift that, since its original
certification, has continued to meet the following:
(1) A maximum takeoff weight ...
(ii) 1,320 pounds (600 kilograms) for aircraft...
(2) A maximum airspeed in level flight with maximum
continuous power(VH) of not more than 120 knots CAS...."

This certainly uncovers some of the confusion. We need to make sure and
distinquish between "light sport aircraft", "experimental light sport
aircraft", and aircraft that can be flown by sport pilots. The original
question asked about a homebuilt, and specifically an RV-3, so I assumed it
would have an experimental certification.


My understanding is that it would be possible to take an experimental
aircraft, and modify it so that it would meet the limitations of the sport
pilot.

Not if it had an "original certification" outside the
limits. I suppose there's an opening for taking it apart,
and building something new from some/all of the parts, but
that's more than just modifying it.

I don't agree with this. I've certified 3 experimental aircraft, and not
one single time has the "maximum airspeed in level flight with maximum
continuous power(VH)" ever been listed anywhere. The Vne is listed, but
many aircraft have Vne's that can't be achieved in level flight, so it
doesn't mean the plane will do it.

The operating limitations received from the FAA with the airworthiness
certificate don't list ANY speeds, as these must be determined during the
phase one test period, and noted in the aircraft log. The log can be
modified over time to reflect changes in the aircraft as well, so if
modification is made that would reduce the weight, airspeed, etc, it appears
to be perfectly legal to change the log to reflect that mod.


That does not look like the "exact question" asked above.

Let's see... The question I asked AOPA was- "I currently have an RV-3 with a
two rotor Mazda engine. If I put a single rotor engine, with far less
power, and use a prop that will reduce the top speed to 138 mph, could it be
flown by a sport pilot?" Seems pretty close to the exact question to me
:-)

Cheers,
Rusty