Power loss
 

So recently I had my little 49cc(bored out to 75) brought in for an oil change and tune. I started riding it and it seems to have lost alot of power all of the sudden, also before they worked on it I was able to freely rev the engine up to whatever rpm I needed for power. It seems like they may have replaced the piece that controls throttle on the engine (part looks brand new) and now it won't throttle past 5000 rpm. Any idea what the fuck they actually did and how I can fix it? I'm not super mechanically inclined but I'm not stupid either.
Thanks