power for pw 50

I got my brothers pw 50 running and it has no power it won't even pull my 55 pound brother up a hill and it used to pull my 130 pounds around. I've already cleaned the air filter and sprayed some carb cleaner in the carb. Is there anything else i can do to get this thing back to power. BTW when i was reading through some of the other posts people would ask if it had been sitting for a while. does this affect it and it has been sitting for 2 years then we got it out again then after a couple months it sat for another year. Any Suggestions!!!

Okay i took it off dissasembled it cleaned it and reassembled it, and it still has no power, i also put a new spark plug in it, btw thanks for the advice indy

okay ill try to get the parts and get it running good again, thanks for all your help