The 350 was good for 65 or 70 with the wind at your back. DAMHIK
The 500 is supposed to do 80 but I've never ridden that one, my 500 wasn't too happy over 75.
There is a 13 hp diesel that appears on Ebay now and again and it would be the best choice as long as the bearing set up is OK, those Chinese diesel vary quite lot. Some very poor some almost as good as the Yanmar.
Either way a good fairing like Craig Vetter is flying would be the way to go.