The willow tree is located on a hill in a valley (Central EU) known to have strong winds and wind gusts of over +70 km/hr blowing from North West. Four years ago i removed the front trees & bushes so the williow tree became more vulnerable to direct winds and as a result, most of the front colas facing NW broke by time.
First major crack in tree trunk occurred on the last week storm, while today's storm & winds increased the thickness of the crack.
Shall i reinforce it with metals and chains or Cut it down and hope for it to grow again?
I would drastically cut branches on one side , then with MUCH less load on the split , put one or a few threaded rods-washers-nuts through the split to hold the trunk together. I bolted together a Chinese elm this way . The tree was only about 10 in. ( 25 cm) diameter ; in a few years the bark grew over the nuts .
Given the location and severity of the crack it looks like replacement would be the way to go. As you are probably aware, willow roots from its own branches and twigs fairly readily, and also will sprout out as a result of heavy pruning. So it is very easy to generate your own replacements from cuttings. It's not however reasonable to expect a good mature tree to result from simply cutting back to the ground and training up one of the volunteer sprouts, since it will be constantly weak due to the connection to the old stump and growing in what must be exhausted ground.
Keeping the tree would probably involve a combination of heavy pruning (to reduce wind resistance and weight of the branches) and cabling and woodrods (to help secure the tree in windy conditions). Woodrods are simply long threaded rods that pass through heavy branches lower down in the tree, a nut and washer placed on the ends and tightened down; the split is prevented from getting worse in the stretch of the rod. Cabling takes place higher up in the tree to pull major branches together, so that each side of the split is mutually supporting the other.
take cuttings and plant them up in a nursery
take some weight off the heavy branches
cabling if required
plant out rooted replacement trees from the nursery in clear ground in the same or different location as the old tree according to need