Faulhaber 1724 should fit. It is the same motor as the rapier II or the 1717 I believe is the model the dragonfly uses. The link I posted for surplus motors is a US sales website. eBay again is always an option.

Graupner also makes a model 180 speed that is supposed to be about 17mm x 18mm that might fit. They run about 10$ and have a max voltage of 7.5 volts roughly. But I wouldn't guarantee that it would fit. I have no experience with this motor at all.