The HRC piston bearings for the 14mm piston pins are long gone. There are a couple alternatives in the bearing market but they are very unreliable so you will have to reuse the ones you have or search for a NOS part.
There is one very good alternative though. It's the below Yamaha little end bearing that is originally oriented for smaller capacity two stroke bikes. I've used these bearings myself with no ill effects and I found them to be very reliable and can withstand high revs without problems at all. The part number is 93310-21403.