Yes and no, there was a "diesel" cyclemotor or two but I don't think they would run on straight diesel fuel. Usually some mixture of kerosene or paraffin and two cycle oil I believe. The power was quite poor, even for a cyclemotor. I think for the most part diesel engines are just a bit big and heavy for moped applications. The smallest true diesel engines being mass produced that I know of are generator size, probably about 350cc.