I'm with opus.
Although they might not be extinct. They are rare, with the last ones being installed 20 years ago. The only ones that are in use around here are on gensets and the oddball truck.
I'm not so sure about the 500k mile engine statement. Although I'm sure some have hit that mark, most that I've seen are substantially less then that. They are pretty unique in the way they operate, and I've always found them to be somewhat finicky.
So unless you're pretty well mechanically inclined, or know somebody that is knowledgeable about them, I'd avoid them.
There is a very knowledgeable community regarding the two strokes over on 4bt forums that can help you out if you still desire the undertaking.