Don,
   I have an OC-3 that I rebuilt the motor on but has been sitting for several years and has never been fired up since the rebuild. I think the transmission may be messed up as I think it locks up in one or more gears. How different is the Rear of the 3 and 4 ?  With a cursory look I do not see how one would get at the gearing of the OC-3, unless it is through the rear. I am not sure I am interested in getting so involved in this little machine.... it just sits in the barn waiting......
     Got any snow way out west in Loweville?  None here in Maine.
    Cal