Help identifying old pro ama yz125

Looks like a privateer rider with some backing

If you want better information and feedback, you need to take better pictures.

Look for a modified frame; bracing, extra tabs for added rigidity/strength, modified radiators for better cooling, titanium hardware, drilled bolts for lockwire and any engraving on the engine to help identify the tuner. Also look at the carb closely to see if it has been bored, and maybe the choke circuit plugged up with epoxy. The clues are there with things they couldn't swap back out for OEM before flipping the bike when they were done with it.

Wheels often have id markings on the hubs... the pipe could have id markings on it... a name written in marker pen under the seat or fuel tank... check the cdi box while the tank is off too, there are always a few things to look for, looks like a good base for a build though...

It's definitely not a YOT pro bike. They didn't use those clamps and they always cut the arm off the fuel petcock.

I bought a 2003 YZ250 from a guy with the same AMA sticker on it. If you can find somebody at either DMV or Yamaha Motor Co to look up the VIN they can tell you who owned the bike. I thought my YZ250 could have been owned by a factory racer, but it was just some privateer guy that bought a bike from Yamaha and raced it in AMA Supercross.