Make sure that you follow a few purchases online, so you know what the market will bear.
There are three Gio big wheel bikes: the X31, X35 and X37. The X31 is the latest version of the original Gio dirt bike, and it has a Zongshen engine and a 37.4" seat height. The X35 has a Loncin engine of the same design as the Zongshen (OHC air cooled Honda clone), and it has a seat height of 35". The X37 is a liquid cooled bike with what appears to be an NX250 style engine with some mods, and it has a 36.22" seat height.
Check 'em out at
http://www.giobikes.com/Dirt-Bikes-in-Canada/
FWIW, I'd buy the X31. I'm a Zong fan, and I have no use for liquid cooling in a 9.5:1 compression ratio engine. Too much needless complexity, IMHO.