Kubota B6100 Engine Replacement(s)

[Drpower]

I have a kubota B6100 with a d650-A engine. the engine has over 6500 hrs on it and i rely on the tractor daily. I am trying to find a rebuilt engine, or one with low hours that could, if need be serve as a direct fit replacement for the current engine. I was wondering if the D750, or any other kubota engine is the same size as far as would it fit into the exact spot that the engine is now. Its still running good, and isnt blowing excessive smoke, but if something was to happen to the engine i have to have something to replace it with.

Any advice or input is greatly apprieciated.