So, my lawnmower is an old Briggs and Stratton self propelled walk behind. I used it for cutting grass when I was a kid, then mothballed it under my parent's house for 15 years or so until I bought my own house.

It uses the gear-cog type wheels to propel itself - chain driven cog engages with the wheels, and after a couple decades, the wheels and cogs were pretty worn out, so I figured I'd buy replacement wheels and cogs.

Apparently, they're hard to find, but thankfully I have the internet.

And a very old lawn mower.

 

The old reel type mowers are still the best and I'm sure you can find the parts for yours somewhere. I would start with a search using the brand name or model number.

 

You need one of these
http://www.mowersdirect.com/Ariens-WAW1034/p2253.html

I just picked one up and love it...10.5 hp Briggs...cuts my grass in half the time and looks professional