It all depends on how much work you waant to do, the easiest option by far is to get a standard Mini engine that will bolt straight in. A 998cc is plenty fast enough around a farm & they normally go quite cheaply, try Fleecbay or better still one of the many Mini Forum sites. However if you do decide to go K-series it is possible but with a lot of work, Along with the points mentioned above you will also have to work out how to operate the clutch, either convert the frame to cable & then try to get a long enough cable or convert the gearbox to hydraulic, my preference but still work. Then the biggest hurdle is to get the gears to actually select as these are rod change but on the wrong side of the engine. You have a lot of work in front of you to get it to work, personaly I would just stick with a cheap Mini engine & get out there & enjoy it, if you go K-series you may still be waiting to drive it this time next year.
Alan...