You will also find an additional engine drain point on the crankcase left hand side under the primary drive case. Remove the A26 brass plug to get rid of oil in the bottom of the crankcases.
Access to the oil filter is gained by removing the magneto cover. You will find a large brass plug, PR4A. You will probably need a large socket or box spanner to remove this if it has not been moved in a while. A little non setting compound on the thread when replacing is useful.
The original filter was felt. Modern cartridge paper filters are available which you may find more convenient.
Another tip regarding oil is to run the engine for a minute or two before checking the oil level in the tank. Many, including myself
, have topped up the oil and then had an overflow over the top of the engine when oil in the sump was returned to the tank.
Its good that you're getting to grips with it.
The club shop stocks A22SS/DRAIN which is the short banjo bolt with built in drain screw mentioned by Bruce. The metal/ rubber sealing washers are known as 1/4BSP Dowty washers in the UK and I buy mine from the local farm shop. John Deere, and other, tractors use them on their hydraulic lines. You'll need to change the oil filter as well as the oil.