I built these. I used old ring gear but you could just grind some teeth to fit. They work great and take about 5-minutes to install if you have to pull starter and about 30 seconds if you don't.

http://good-times.webshots.com/photo/2657411170102454676xaEfmI
http://good-times.webshots.com/photo/2257528360102454676zxeIoi