I'm starting to re-think my practice of running these derailers into the ground, as the unwanted failure could occur at any time, which could actually be dangerous or at least damaging to other parts of the bike, depending on circumstances. Think about it...

If a crack is repaired, how strong is the repair? Wouldn't it be cool if there was some way to run a threaded metal repair though the cracked area?

One thing about replacements is that sometimes the clamp diameter is actually 28.6mm instead of 28.0mm, so make sure the clamp closes with some discernable gap present where the clamp pinches shut. A little filing will fix this if needed, as will a thin shim of (preferably) metal. A plastic Simplex replacement could also be 28.6mm, which should not be clamped on a 28mm seat tube without shimming first (to prevent unwanted flexing stress).

I would think that a Campagnolo Velox derailer would make a suitable replacement, but the Simplex and Campagnolo cages may not be interchangeable because the Campag derailer's push-rod is tilted on some or all of those.

I scored a pair of 1961 Simplex "Export 61" derailers, which are near-copies of the plastic ones, for possible use on my Steyr Clubman when the time comes (any day now, cracks showing!).

I've had much better luck with the plastic rear derailers myself, but have seen the limit screw bosses crack and even break off. I'm currently still using cable tension to control the hi-limit travel since the boss broke off during a tip-over 20 months ago.