You can take the speedo mechanism only out of the plastic case it is in (there are two brass screws at the back) and you can just replace this with the other speedo mechanism and surrounding ring with the km/hr on it the electronics type stuff is left as is because you only change the speedo and ring nothing else. The plastic case is the same one you used previously
The screws I mean are indicated by the purple arrows below:
You will have to take off the glass and front of the speedo guage to get the speedo mechanism out but that's easy and I don't have a piccie of it
Oh sorry I just reread your post...
Do you mean that the front face of the speedo you are putting in covers up the warning lights or it just doesn't fit. in which case I would be checking whether the rings with the mph and kph printed on them line up(60miles is at about the same place as 100km and zero is at the same place on both) and if they do (which I suspect they probably do) just change them over although some of the clubby speedos had miles/hr and km/hr but the miles were on the outside ring so if your metric speedo does that then it wouldn't work
Sorry this sounds confusing
