WACO,
I have a YM1110D on which the shutoff didn't work. The lever is that cheap "pot metal" junk. It has two little prongs that engage a plastic cone shaped valve assy. It also leaked a little. It is a very simple arrangement. All you have to do is remove the little screw on the top of the fuel bowl assy, and the lever will come out - but be careful, because there is a little spring in there as well. It won't fly apart - but remove the lever slowly so you don't lose the spring. There is a little o-ring around the shaft where it runs inside the fuel bowl assy. You should be able to get a replacement almost anywhere that sells this kind of stuff. The parts manual lists it as a P-10A o-ring.
I scanned part of my parts manual from the YM1510D and attached it here. The o-ring is No. 7 on the list. I think this filter bowl assy was used on all YM series Yanmar tractors. If you can't get the o-ring from anywhere else, you can get it from a Yanmar dealer, since it was used on the U.S. Yanmars like the YM135, and YM155.
Hope this helps.
Greg