I only mention this website because the engines seemed to be from reputable manufacturers and I didn't see any clones. If I was going to build a generator based bike I would, personally, prefer to use a good secondhand engine from a known manufacturer rather than buy a same cost new clone from an unknown manufacturer.
I had a look around this yard a while back and these guys always have a selection of Diesel engines. Some were Hatz twins on pallets, others were small units attached to vibrating plates and all sorts..
They might be worth a phone call if you are UK based and looking for an engine? They have a few more than are listed on the website, I'm pretty sure of that having walked about their yard. They are based in Devon.
http://www.woods-group.co.uk/
Small selection of Diesel's here...
http://www.woods-group.co.uk/pumps.htm
Woods Group Surplus engines, (huge) turbo's etc
Moderators: Dan J, Diesel Dave, Crazymanneil, Stuart