Thank you both for the hints. I`ll change with a motor of 1/3 hp - I succeded in recovering both the mount for it and the blade too (Torrington - aluminium 18in diameter 5 blades, pitch approx. 35 degrees) . Comparing with other blades found on Internet - 3 blades and lower pitches at comparable diameter and rpm it looks like it needs a powerful motor so I`ll keep it at 1/3 hp. I`ve seen other similar units on the roofs around and it looks like the original blade this Torrington one. Can`t imagine this blade working with 1/5 hp, at least not at 1075 rpm. Tomorrow I`ll search for a new motor, however I`m thinking firstly to check if maybe worthy just to rewire this old one. And by the way - here too Johnstone would talk only with a dealer.
I`ll come back with new details.