Hi!
All .40-.46 two stroke engines on the market ,except Rossi and MVVS, has those ball bearing sizes! One metric one inch! MVVS has the same at the rear but a smaller metric up front , Rossi.40-.45 has a larger metric at the rear and a larger metric up front.

Here is a picture of how one of the best bearings look like!
From Swiss (WIB) with C4 play, for best reduction of friction and with phenolic retainers.
These bearings are used in racing engines like Nelson and JETT (and some MVVS engines) and could of course be used in a sport engines too.