Having owned a RS125 and ridden it mostly on tracks, I doubt very much that it could do anywhere near 100 mpg.
Triple digit speed maybe, but not very safely, nor is it good for its engine.
They were a few street legal verions sold in the US, even the 250cc 2-stroke versions, they pop up once in a while on eBay. But neither are good choices as a dependable street purpose machines IMO.
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