They actually make a small diesel based off the Honda GX series, they are hard to find nowadays due to the stricter EPA regulations. They are known as 168F diesel and mostly sold directly through Chinese distributors so watch the shipping costs, they're usually more than the product. There was a dealer in Michigan who sold them many years ago
carrollstream.com but I am uncertain if they sell them anymore, they may be able to get one for you though, it wouldn't hurt to try, give them a call 1-888-208-2088.