The 3rd gen Honda vtwin Gx630-GX660-GX690 have been out for 2-3 years now, did someone try to run them or do some R&D.
Basic info it is a 90 degree vtwin, 42cid, hemi style combusion chamber, steel rod with bearing, forged crank shaft, Digital CDI ignition
I have done some research, it has some big problem with them.
-the head and cylinder is one piece, no head gasket, installing bigger valve may be impossible or bore them for more cubic inch
-the cam gear is plastic, so some kind of billet gear might be necessery, the cam do not use conventional lifter, its a rocker and the cam only have 2 lobe
-The Ignition is digital CDI, but they says it has a rev limiter build in, so if its like the others new engine its around 4000rpm, so others coil or electronic ignition need to be use
-Of course no billet flywheel for that engine , maybe one from another engine could fit...
More info would be nice, i have this in my head since i worked on a new generator using this engine this morning