Thought I'd add to this old thread just for posterity if someone in the future should Google for this engine.
I run the geared GO-435 on my aircraft and as mentioned, they're no longer supported by Lycoming, and haven't been for years. There's a joint called Central Cylinders in Omaha that bought all the gearing, surplus parts etc and they are the biggest overhauler of these and many other geared engines. Parts are still quite easy to get hold off, but getting scarcer. The overhauls are very expensive simply because CC insist on tearing the gearbox down in conjunction. Now, you don't have to do that if the gearbox is in good shape, so you could bring the engine to another shop and get away with less. Problem is not many places still want to deal with the geared ones, so you're choices get slightly more limited. But there's still quite a few around. A good way to determine if the gearbox needs attention is to test the play about 4ft out on the prop - if it has more than 1" play, it's probably time.
The GO-435 is a pretty solid engine that is basically a 6-cylinder O-290. Most have low compression, so can be run on Mogas and were certified for 81 Octane fuel. Low compression also means longer lasting. The gearboxes have a reputation for being bad, but that's not really true. If you fly them right (positive drive and don't throttle jockey too much) the gearbox will outlast the engine. The GO-435 only has 1200 TBO, and that's a reflection of the way the engines were used by people used to flying direct drive engines. There's nothing inherent in the design that says they won't go quite a bit further if treated well.
Adam - Transplanted Euro guy with legal papers and licenses. JAA and FAA PPL ASEL AMEL, Aero Commander 520 1953.