Interesting that 100 years ago in 1904 the first known diesel motorcycle was built;in Holland I think.
"The Doppler" and it had a "Brons" motor.

Isn't that Enfield built in India with an Italian engine?

Another thought for the future - BMW make diesel engines for more and more of their car models - BMW also make motorcycles.
Given the advances in diesel technology and the rising price of fuel it is not impossible that they could produce something reasonable.

Britain is currently supplying advanced diesel technology for motorcycles to equip the US army.

Will we see a diesel class at the TT?
Many old Bedford bus drivers would have tears in their eyes if they could hear that classic rattle as they accelerated away from the start!