Have read the following inefficient solution, but nevertheless one solution in an Oz magazine:

Leave the old wheel on and get someone to turn a wooden pulley for you. The wooden pulley needs to be connected to the wheel of the sails and will drive a generator from an old tractor. Not a modern alternator, which requires higher RPM. Such a tractor generator will turn like a motor when the batteries are connected, so you need a switch to only switch it on when the wheel is turning fast enough and is generating power. Or a better solution are diodes to ensure the current only flows one way!

The diameter of the pulley will need to be calculated, but I suspect it will be about a 4 : 1 ratio to the diameter of the tractor's generator. This generator will have carbon brushes which will wear out so the first thing you have to do is change the old brushes and perhaps also the bearings.

Not many watts but enough for a few lights and perhaps a lap top with a 12V interface. A large enough car battery should be OK for this experiment rather than a deep cycle variety.

Old mills like that need to be manually furled out of the wind when storms are expected, so watch it! Many a windmill lost its wheels during cyclones (hurricanes) in Australia.

Have fun in Malta!