It's a bit trickier then David. I take it the big and small belts are different widths - otherwise you'd have said different lengths. That means the double crankshaft pulley is a special one with two different-sized V-sections. Also, there's no reason why the two pulley sections should be the same diameter. The calculation we discussed is for the pulley section that drives the cutter. To size the pulley section that drives the wheels we need to know how many turns of the wheel-drive pulley it takes to move the mower forward, say, three feet.
When we've worked out a design for the crankshaft pulley you could ask around on this site for a pulley fairly much like that. If you can't get one, you'd have to have one turned up on a lathe. Not a big job, but you'd have to make a detailed, dimensioned sketch.
