That should be a lifan, or some other clone. Great motors, lots of options! I believe Silverbear, or 2door would be the one to ask, if I remember correctly Silverbear has put several into the larger or modified stingray chopper frames...
Search the 4stroke section under "alternative fourstroke motor list" or something, I forget?!?
But with a lil' searching, there's a TON of info on here!