Well NO you cannot put a 125 swingarm on it...BUT...you can put a big wheel 85 kit and extended swingarm on it. Especially if you look for an 85 instead of the 80.
A suzuki RM85L is the big wheel version with extended swingarm. This kit is avail for KTM, KAW, hon, Yam... Then you Can get a 100,105,112 kit for it and run around in supermini trim. Until you are read for a 125 this is an excellent option and you can find them already done 1 or 2 years old from an ex racer. Sometimes for a great deal. I know we have about $7500 in our supermini but it would probably fetch about $1500-$2000 now. It would make a fine bike for someone not trying to be competitive at a national level. But we are saving for my younger son before we get him a new supermini.