Another option is to buy a pair of new sprocket plates and machine your original sprocket hubs and the new sprocket plates to be fit and welded to each other. It would be a simple process for any machine shop, maybe a couple hours or less of labor.
I've done it on my rear sprockets, the small ones. I converted them from original 12 tooth to 18 tooth.
Hint: If you choose a sprocket with a number of teeth divisible by 3, it will be much easier for the machinist to chuck it into the lathe, so less cost to you.
The ASE version of the sprocket pitch is 80. Use the "A" identifier for a sprocket with no hub, just a small bore. And then choose the number of teeth you want. Just a quick search for a 80A36 sprocket on eBay comes up with a 36 tooth TRITAN sprocket for $55.10 plus shipping.
Observe:
http://www.ebay.com/itm/TRITAN-80A3...669141?hash=item1c858c9c95:g:JW8AAOSwlndZGy7O
Here're a couple pics of the ones I made: