Thanks to a benefactor (who wishes to remain unnamed lest he be condemned for helping me perpetuate the profanity of Cyclone derailleurs on a Masi frame), I now have a functional Cyclone RD again. He sent me a pulley cage spring.

It turns out that installing it was quite simple. I had opened up the derailleur by removing the end cap from the pivot shaft but did not remove the shaft from the cage. The new spring slipped down over the end of the shaft. After a small amount of shuffling around, the inner tang dropped neatly into its hole at the bottom of the well. I removed the stop screw from the cage so I could twist it around beyond its normal stop point, then pushed it outwards into position until the outer tang of the spring fit into one of the slots in the end of the shaft. Once the end cap was popped into place it all stayed together nicely. Then I just torqued the cage back around and replaced the stop screw.

So the bike can be ridden again. Time for some serious test riding.

Then paint. That's a whole 'nother ball of a different color.