Like Eddy, I'd be fairly confident they're close, but cannot guarantee it. They do have the same 61mm bore and the cylinder base gasket shows the ports to be in a similar position. Does that make them identical????
You do know, though, that LJ50 piston kits are still available Ex Japan:
https://buyee.jp/item/yahoo/auction/g423385148https://buyee.jp/item/yahoo/auction/x695856665Not as cheap as Chinese pistons, but the set does include rings, pins and retainers. I don't know how much you're paying for LJ20 pistons, but for complete sets, I don't think they're outlandish prices.
BTW, Buyee puts a block on any engine related parts, as they cannot ship anything oily or smelling of petrol. But for clean, new parts, send them a request to unblock, and they usually agree.