Martinfagan

i have k6 bandit. engine is passed redemption 96k. will mk1 or mk2 engines fit into the frame without much fettling? i can get mk1 or mk 2 engine for aroung £300 but one from k6 is over £800. Bike still runs strong but have noticed oil leak from lower engine where it meets cylinder head so think it may be quicker and cheaper to replace engine. Bike was previously used for daily commute from Northwest to Bournemouth hence the mileage