Nice videos but I have a suggestion, if your going to run these engines without a radiator, supply a little water to the water pump so you don't burn the pump seal up. Some of those seals only take a few seconds running dry before they are damaged.
Another thing to look for when purchasing an engine for your project is to make sure it has a water pump. Some of the Kubota engines use thermal siphon to circulate the water and will require a larger radiator or the addition of an electric pump.
The z482 Kubota and 2TNE series Yanmars are excellent engines but I personally feel they are too small for your application. The Kubota Z600 or Z620 would be a better choice. Still two cylinders but with a good bit more horse power.
Lengthening the frame for the engine you have may not be as daunting a project as it first appears. The hood is usually the thing that requires the most attention.
Good luck with your project. I will be fun to see what you decide to do.