"IS it this one your using a metal fan in?"
Yes, I'm using a metal fan in my Sanitaire, bought from an ebay seller (see link below), I do have spare plastic ones, including the one it came with originally, but, I prefer the metal fan...
As for spitting out cat litter, I don't have the same problem, but then the cat litter is out of the way of any carpeting, and the stuff we use is pelleted sawdust (Cobbycat, check your local petshop, it's good stuff and absorbs smells nicely) that breaks up on impact, but, it could also be that you have your height adjustment set wrong, maybe a tad too low, thus restricting airflow which means the thing can't suck up the litter through the fan, and so it bounces back out...
As for curved blades, I think we've done this one before, there's no fan for Sanitaires or Eurekas like ours that have curved blades, and that one in the link would rotate backwards, and Kirby & even Hoover fans would also turn the wrong way, so to put it lightly, you're stuck with straight blades... :\
And last bit, use Hoover Senior (Convertible over the pond) belts, they're tighter than the normal belts and thus don't burn off as fast, I've been using them successfully for a while now, even so much as the first belt I bought last year was only recently changed, so I thoroughly recommend them (for the residents of the US though, do NOT use them on Blender motor models, they will damage the bearings in those motors)...
Link to the metal Sanitaire fan:
http://cgi.ebay.com/160586319443