Hi there,
I'm pretty new to Douglas motorcycles myself, but I found a clip on youtube of someone having rebuilt a Dragonfly
and I think it's this company here
http://www.classicsuperbikes.co.uk/ but they don't look cheap. I'm sure you know you can get gaskets and probably a new clutch from the club, and having removed my engine and gearbox and split them myself recently, it's really not that hard. There's also a post on here somewhere where I think it was Eddie recommends filling the bellhousing with petrol to clean the clutch disc.
On the wheel front I *think* I read or had a conversation whereby both your style wheels and plain silver were either an option or an age variance, but don't quote me on that. My Dragonfly (also in black/silver) just has plain silver wheels.
For tyres, sometime just buying new tyres in the same style can help if your original tyres are looking as old as the bike.
http://www.vintagetyres.com/ have a good reputation amongst classic car owners so it may be work just giving them a call and having a chat.
Hope this helps a little
Andy