The RS50 is street legal. They run about $2-3k used. There is a 75cc kit that can add power.
The RS250 is not street legal. However, some have been "made" street legal and have titles. There were some early 'grey-market' imports that came with street equipment, but the official imports are race-only versions. There is a kit that can boost rear wheel output to apx 75 bhp.
If the RS50 is considered a scooter and you added the 75cc hop up kit, then it would still be legally considered an RS50 scooter. Nothing would change on the title, therefore it would not be treated differently.