The O-435 is obsolete and an orphan, yes, but so is the Merlin and P&W R985... There are still plenty of parts around, they're just not at Aircraft Spruce. I know a guy who had boxes of BRAND NEW cams, followers, and other stuff for the 435. There are several good flying O-435's on my airpark and they make good power, run smooth as butter and are reliable as a stone axe. I'm looking to build one for one of my future projects. (I'm going with 320 cylinders though).
If you're looking for the level of support that you get for the modern engines, then yes, run away. On the other hand, if you're willing to do a little digging for some parts (if needed), and you get a good price, it's a fine engine.