Good finds! I’m not sure I would agree that the design is simple, but it is elegant and well thought out.
I don’t have my P-34 handy, but I think the mouthpiece is the same as for the Pianica 32A. If so, a mouthpiece from a current model (P-25F, P-32D. P-37D) will work, though it’s a tight fit. But if you haven’t tried playing without a mouthpiece, try that. I like playing these without one. For those who are looking for a current Yamaha mouthpiece, I usually suggest just buying a P-25F if you don’t already have one. Everybody should have one!