Jonathan,
As Chris said, decent conrods are the hard to find part for the 500/600 Aero engines - but it is possible to recondition rods with knackered bigends. Chris supplied me with a pair of such rods several years ago when I was desperate for some to go into my sprint motor. I felt sure there must be a way of bringing them back to a usable condition - after much measuring and comparing with postwar rods, I figured it should be possible to liner the bigends, much as Alpha's used to do for the Dragonfly. Having searched through my scrap box, I found a couple of liners that had been removed from Dragonfly rods during modification to needle roller bigends. These liners were about 30 thou smaller than the bigend eye of the Aero rod, so would stand being reground. It was then just a case of determining the best diameter to bore/grind out the rods to give reasonable strength with regard to both the conrod and the liner. At this point, I also figured that using 6mm rollers instead of 1/4" would result in a stronger liner. When fitting these liners, it is imperative that the bigend bore in the conrod has a good finish and is perfectly round and parallel, so that it gives the liner the best possible support (this is where some of the commercially available reconditioned units fall down, resulting in the liner flexing and eventually breaking up!). These reconditioned rods have now done 4 seasons of sprinting and the conrods are as good as the day I fitted them - the only bigend problem the bike has suffered was a broken bigend cage.
Hope this is of some help,
Regards,
Eddie.