It's always dangerous saying that Super Tigre didn't make a particular engine.
As a general rule I'd say that if a particular configuration was possible then they would have made it for some group somewhere.
The G20/15 series was on sale from the mid 60's to the late 80's, ample time for all the possibilitues to be tried.
I have an unrun G20/15D (2.5cc diesel) with a factory fitted r/c carby in my diesel stach.
I've also heard that there was a .19 version.
Looking at the ebay item I noticed that it's from
toys_tophobby a reputible dealer who knows his stuff.
I'd be happy to buy the engine if the price was right, but I'm sure that it'll go through the roof.