If you are being serious, then write serious:
"I have a 12 inch bike from Wal-Mart that I am looking to try to just have some fun and screw around on. You know, a pit-bike thing... nothing serious. But, I wanted to upgrade some stuff. Do you know what would fit?"
Well, if it is not a 20" bike then you aren't going to get good forks, rims, or tires for it. But, otherwise, everything should be close to standard. The hubs are typically 3/8" so any 3/8" hub can replace the existing one. You could put a $200.00 Profile cassette hub on it if you want.
But, really, almost all bike stuff across the spectrum from pro bikes to k-mart crap still uses standard sizing and fairly common placement. So, a lot of stuff to trick it out can come from bike shops... Still not sure why you would want to because for the same money you can buy a GOOD 16" Hoffman or Haro and really have something that deserves to be tricked out.
When you spray perfume on poop, it's still poop.