is the problem that you are having either that you do not have the correct spanner to fit the G23 nut, and that you are having trouble locking the gear box up so that you can undo the nut or both?
The nut size is one inch whitworth, because of its size the cheapest way to get a spanner to fit it is to buy a box type spanner( the old tubular) type . A 3/4 drive socket with extension bar and handle would be very expensive .
To lock up the engine, I use a length of number 2 nylon rope shoved down the plug hole, it sounds horrible , but it is simple , and works. With the spark plug removed, turn the engine backwards until the engine comes up to compression. ( both valves closed) then turn the engine forwards until the piston descends shove the rope cord down the plug hole ,and turn the engine the engine backwards until the piston can go no further. the piston is now jammed against the compressed rope, and as both valves are closed no damage can be done. T.
To tighten the nut, set the engine up the other way .
hope this helps.
