As RJ said, you could conceivably cut and rejoin the windings to get the full power of the transformer- here is a markup of your schematic showing the basic idea. Splitting the winding and putting the two halves in parallel. Forgive my sloppy picture editing attempt lol. Imagine that the two windings are the same number of turns (which they probably are already) so you just have to find the point on the winding where the fan connects and make the cut, then reconnect using solder and big copper lugs. Need a big soldering iron or torch, that wire is fat. Mark S.