Yamaha 20/25 gearchange

I recently bought a CJR 14 at an extremely good price with the bonus of 2 engines thrown in which both ran but could not select gears. The 4hp I have sorted with new oil seals, bearings and a serious gunk scrape out. It runs a dream. The20hp(on the identification plate),25hp(on the engine casing) is, I think, a simple fix if I new how to get at the top gear change linkage. The external operating lever goes through the lower outer casing and is linked by a rod to a lever whichgoes down insde the casing and feeds across under the front of the engine to the top of the rod which travels down to the gearbox. Its the top of this rod I need to access as the rod pushed manually at the bottom of the leg slips into neutral and both gears beautifully. The external lever through as far as where it disappears under the casing is all wear and play free. The only way I can see to access the problem area is to remove the power head and I can see 6 bolts going up into it from below, but I am wary of starting as I am unsure if there is any other fixing I have missed, or any potential minefield waiting to fall to bits as I move it. A friend has told me that the 20hp and 25hp were the same but with different carbs, thogh its always possible that the engine cover was replaced with the wrong one. Its a model 20CM, number 6a9 003093. Can anyone help,and does anyone know what year this engine would be?
Frankiepoop