From the manual:

Ideally, when laying up the engine for the winter or if it is not to be used for a considerable period, the engine should be removed from the boat and stored in a cool dry location, suitably covered to protect it from dust and dirt.  If this is not possible, the following procedure shuld be
adopted:

- 1. Remove the air filter, start the engine, and spray protective oil (Part No. 50 173) into the inlet manifold.
- 2. Open the plug on the cylinder block and the plug in the sound absorber. Ensure that all the water drains out.
- 3. If protective oil in a spray bottle is not available, proceed to the following four stages:-
---- 1. Remove the carburettor, spark plug and V belt.
---- 2. With the piston at Top Dead Centre, pour about 140 mls of  oil through the suction point into the crankcase
---- 3. Turn the piston down to Bottom Dead Centre and tilt the engine so that the oil can flow onto the bearings.
---- 4. Turn the piston again to its Top Dead Centre position and pour about a spoonful of motor oil  through the spark plug hole into the cylinder.
- 4. Rotate the crankshaft two or three turns, leaving it in mid-stroke position, so that the cylinder ports are covered.
- 5. Replace the spark plug and grease all exposed metal parts.
- 6. To protect electrical components, spray them with a suitable water-repellant spray.
- 7. Clean the battery, check the electrolyte level in it, and charge it.