i'm wondering if adding some alcohol to gasoline will do it .. small gas tank won't require much.
The ideal straight alcohol air:fuel mix ratio is near 7:1 or so .. And straight alcohol requires much bigger mainjets.
Maybe if the right gasohol blend were found it would make up for less available oxygen at altitude (someone would need to do the math )
and since adding alcohol would raise the octane rating, compression ratio might be raised a bit to offset slightly poorer performance with the blend .. but there might be other things to consider like a high alcohol percentage blend reacting with some rubber or metal parts..
maybe adding alcohol is a way to artificially test the effects of running a smaller main jet .. leaning the mix at sea level or wherever you happen to be..