I can't tell you the sizes, because I'm not sure what measurements they require for a tapered bearing. I do have a spare set of steering head bearings, and I can give you the identifying marking from the bearings. If you're going to a bearing shop for replacements, they may be able to cross reference their suppliers with the identifying info from Yamaha's bearings (which are Koyo and NTM bearings) to find replacements from another manufacturer.
Part Number 93332-00078, which is the upper steering head bearing and race, are both made by Koyo, and bear the following identifying info:
The bearing is marked "KOYO", "HI-CAP", "320/28JR", "JAPAN"
The bearing seal is marked "KOYO", "320/28JRR9", "17"
The race is marked "KOYO", "HI-CAP", "320/28JR", "JAPAN"
Part Number 93332-00081, which is the lower steering head bearing and race, are both made by NTN, and bear the following identifying info:
The bearing is marked "NTN", "4T-CR-0643L", "JAPAN YN"
The seal is marked "NTN", "CR-0643L"
The race is marked "NTN", "4T-CR-0643", "JAPAN", "YN"