To my knowledge, high displacement single cylinder engines & the 660 engine are not designed for sustained high revs work (motorway work). If you want to move around at over 70mph for relatively long periods, get a 2 cylinder engine

More than happy to be proved wrong BTW, as i love my 660Z, but i have to forcemyself to keep speed in check ... & replace engine oil frequently.

PS : would be good to hear from original poster what the outcome was !