I'm my Haynes manual it says that the alternator rotor is attached to the left side case cover and it looks fairly simple to remove and replace, the only annoyance is that I'd have to drain the oil and get a new side gasket to do so. I think I've seen the CRB250 rotors on Taobao for around £16 but I would imagine that they are fairly heavy and the postage would be expensive but If I was to get one I suppose I could just tap the original m5 holes with a m6 tap. I thought that solar panels would be a really good idea at first as I could attach them to the top of my luggage box so they wouldn't look too bad but most of the energy goes into lighting which I only use at night so that's a no go :(

A small generator that is separate from the engine could be a better idea now that I think about it; there is some room inside my bike where the air box used to be which is directly above the drive chain so I could connect it to the chain using a small sprocket. If I was to do that would I be better off wiring it directly to the battery, the rectifier or it's own voltage rectifier?

It would be nice to have enough electrical power for a couple of 50w spot lights for unlit roads at night or to upgrade to a 100w headlight bulb just for the sake of it ;)