here's a couple of pictures of the Crosley flat twin engine which powered the pre-war automobiles and apparently the military motorcycle prototypes as well. the motorcycles were also shaft driven. the little twin was made by Waukesha Engine Co. of Waukesha, Wisconsin. it displaced 580cc's and put out around 13hp. from the looks of it it was also an L-head valve configuration. even the Crosley website doesn't give a lot of info about these engines. i'll try and find out more but so far this is it.