K engine code ?

[Fixit]

So my Katana Popup  has an R703 motor, its pretty tired so picked up what i thought was a low (33kms) replacement.

The motors look almost identical but the low mileage motor  has a K75 engine code.

The only differences i can find are that the replacement motor has gold covers and a large retaining washer on the front sprocket to stop the nut coming off.

Does anyone know what the difference is with the two engines ? been trolling the net all day and come up with nothing.

Cheers for any help anyone can offer because im stumped ....