I too have a minigen, much modified.
I replaced the wire with something larger, it's a long time ago so I can't remember what I used, but I filled up the original spool.
I also put different blades on it, and because of their extra weight I had to make a completely different bearing arrangement and blade mounts. There is very little left of the original I suppose, but it gave me a good grounding in messing about with windmills.
If you are interested enough, I'll take it apart and post some pictures, it hasn't been up in the air for some time as it's been replaced by a bigger brother!
I did have it producing 15V in a decent wind, and used this to charge some very small batteries (12v) which I ran some LED garden lights from. Great fun, but whether it is upto the job of charging a tractor battery, I have my doubts.