One thing to take note that RQ Riley one of the original designers of Centurion has stated multiple times is to turbocharge these small diesels. Not only will this give you more power but will also increase fuel economy.
I agree with this but a person must be careful because there is a right and wrong way to add boost. Most people get this wrong with small engines and never really see any appreciable gains that make a measured difference. Adding a turbo charger to a small Kubota engine is not going to do much because you are not going to flow much exhaust to recover in the first place. I think the smallest Kubota engine that is turbo charged is the D1105-T which puts out 32.8hp @ 3000RPM.
This engine was used in the Mother Earth News Kinetic Vehicles MAX car =
Kinetic Vehicles
I think that the goal for Centurion should be a target weight of 1,000lbs. Which means you would need to trim a little over 200lbs from the weight of the car. Next shoot for a power level equal to that of the D1105-T. 1000lbs/32hp = 31.25 Which would give you a similar weight to HP ratio of the SMART ForTwo 0.8 CDI 5AT (54 HP) 1698lbs/54hp = 31.40
Smart car specs =
SMART ForTwo - 2012, 2013, 2014 - autoevolution
This would give you actually better performance than the Smart car most likely because of better aerodynamics that the Centurion has in comparison to the Smart cars .38 Cd.