I have one of those Chinese Yanmar clones, waiting to do a swap into just the right frame. It has to be big and strong because the Yanmar is nearly 100 lbs. Yet the bike cannot be too heavy because it only makes 10hp and I'd really like to see 60mph. 50 will be more likely. My clutch and transmission are a DT200 motor with the crankshft replaced with a jackshaft. This will give me 6 gears and a good clutch. Only problem is to find a large but lightweight frame to take it.
The goal is to make an efficient touring machine that is a bit of a conversation starter. Many of these industrial diesels are not super efficient. Like a Briggs&Stratton, they are more just designed to make rotary motion out of fuel without great regard for efficiency.
As for converting a 2 stroke gas to diesel fuel, it is being done for US Army drones. I think Sonex makes a conversion that uses a special head to burn diesel fed through a carb.