I've written about this in the past of which caused a great uproar. Yet, many in Texas don't want to believe it and use the self propelled argument only to still recieve tickets.
There's a Texas statute which states mopeds, motor driven cycles and motorcycles must be made by certified manufacturers. The certified manufacturers list is updated every 90 days. Bottom line is to fall into one of those 3 categories it can't be self built.
What you can legally build in Texas is this:
551.351. DEFINITION. In this subchapter,
"motor-assisted scooter" means a self-propelled device with:
(1) at least two wheels in contact with the ground
during operation;
(2) a braking system capable of stopping the device
under typical operating conditions;
(3) a gas or electric motor not exceeding 40 cubic
centimeters;
(4) a deck designed to allow a person to stand or sit
while operating the device; and
(5) the ability to be propelled by human power
There's some Texas statutes that exempts this vehicle from having to be on the Texas certified manufacturers list.
So what all that means is it has to be capable of being kick or pedal propelled. And 40cc or less.
Since an 80cc China girl is actually 66cc with that same ratio a 49cc China girl should be barley over 40cc. There's also several 4 strokes under 40cc engines as well. Keep a copy of the Motor Assisted Scooter statute and the statute that exempts them from having to be on the Texas certified manufacturers list with you in case you're pulled over.