Panterra is a cheap chinese company making motorbikes;
the engine is likely off a shelf of a different manufakturer;
so you need to tell us what's the model number.. that should be somewhere on the crankcase.
125cc is plausible, sindce if you google for panterra 125cc dirt bike you'll find many of similar looking engines (mounted to bikes

)
the DirtRunner 125 from baja motorsports for example was a Panterra (relabeled)
unfortunately since parts change during time you need to know the model!
here's the partlist for the DR125 Panterra:
http://shop.bgsengineclinic.com/Baj...a - Red 125cc Dirt Bike (VIN PREFIX LLCH).pdf
(crappiest url ever!!)
Anyways.. look at the picture of the bike in the pdf and compare it to this:
I'm not 100% sure, but if I had to make a guess it's a
Lifan 156FMI engine that you have.
Again check your crankcase for model numbers.
everything else is guessing
'sid