Oh how annoying! I've seen this before when people have incorrectly set up the YPVS servo and stripped the teeth on the gears.
It's probably easier to just buy another YPVS servo sadly than trying to find the individual cog.
https://www.ebay.co.uk/itm/307114030022?_skw=Yamaha+DT+125+R+YPVS+servo&itmmeta=01M0T0CTAKVKYBVERW4KK9HKJQ&hash=item47816c33c6:g:QqIAAeSwkVdqdvO2&itmprp=enc%3AAQALAAABAGfYFPkwiKCW4ZNSs2u11xDV5F88zNLFmQlOj5cc4%2FxlPVfMrS0CX9zdgmCMx0exrIiMBYEnl2C70gEhZcvwr3BYq%2BewLJHyMjxf%2BATBLTUnAq5B4Os%2FAuVyeOUNpVSRYxblUNuwL%2BjqWn0F95mbpkEWkXEDU%2F4blrRDuajNXDPVd8py1aTbWsSecsyFqwFQqqWjrgMG65s52PTjh2LPZ%2FOmWbQsy4dMVcVkq6VX2lCX6cdUUBN6SMPZHm3xNkDwYsJXuOa1TCyyOEaSwcuLKfJrQwb9YP1PtVMNdQS0UqCxAXfBI4hQNMZjPJwu4V6qdKAjffzCnESPO3DSoS0%2FsfE%3D|tkp%3ABk9SR9als8CGaA
If you have access to the a 3D printer, or are capable on CAD, you might be able to 3D print a new set of teeth, setting the TPU to 100% fill should be strong enough to cope with the demand. My 3D printer is pretty darn accurate so it would probably do a fair go at it to be fair.
