Andy,.. I think you are going to be broken-hearted.... I don't know anyone who makes a "True" diesel that size. I think You'd be better off converting a Ryobi 4-stroke gas engine weed eater.
No gad/oil mix,overhead valves,and would probably run for days on a 5-gallon tank. And...they are $79.95 plus shipping at harbor frieght! If You could increase the compression of it to around 22:1 and build a timed injecter system for it,You MAY be able to convert it to a true diesel! Bob