Its all in the bonnet, borrow one off a grey Fergy and size it up. Last year in a show I saw a DB Cropmaster with a Diahatsu Diesel engine, it looked ok. Last night at a local show, a pal and his Miss brought all three Grey Fergys, the latest fitted with a Rover V8, and it looks ok, just over powered, but truly 'Pimped".
Quite sure you could do something original with a Kubota. With reference to the BMC tractors, they are a very desirable machine and have their own club (Morris Minor engine in a little tractor). What Kubota did was add the 4wd system to the small machine.
Ref the Zetor, a neighbour has one like that (from new), only fault it ever developed was in the front axle, a bearing collapsed. Lives out a lot, good machine.