I've looked at the pictures backwards and forwards...and I'm stumped. From the neck back it definitely looks like a cross between a Broncco TX-3 and TX-4, it has attributes of both. BUT, there are also things that make me think it's not a Broncco, like the angle of the down tubes on the front of the frame, how/where they meet at the neck, and of course the forks. However, the engine mounts, foot peg mounts, rear tire mounts and overall frame design looks like a Broncco.
I guess we'll just have to take your word for it that it has NEVER EVER been modified and you've owned the bike since it was bought at the store 10-20-30-40 years ago. If you can't 100% verify that, then anything is possible. I'm not saying you're wrong per se...but quite frankly, I know a lot about Broncco's and this is the closest thing I've ever seen to a Broncco that ISNT actually a Broncco. That doesnt mean it's a Broncco though.
Like I said above, it has attributes of a Broncco, and attributes of other bikes. Anyone who knows Broncco's knows they are hard bikes to find, as they came from Italy and were only sold for 3 years in the USA (1969-1971). I know the importer personally. So, what I am saying is, it would seem highly unlikely another mini bike manufacturer copied their bike...and for this to be simple coincidence is just too much to ask. Having said all that, I still don't know what your bike is. Good Luck!