The Brits and the US had been working on a diesel powered motorcycle since 1993 with the M1030M1 entering service in about 2001 - USMC ordered 440 of them. I'm not a biker, but being able to do 140mpg (120mpg US) and not resting my wedding tackle on 3 gallons of liquid explosive during a shooting situation rather appeals to me.
(Riders report
here)
You might be able to sneak a few into your game world on the pretext of accelerated military developments.