Well, I guess I sorted that shifty business out. Though it is unclear to me how, I can now shift cleanly into all four gears.
I removed the outer cover. Looked intently at the shift lever return spring. Then I removed the stop plate and spring. And, just because, I turned the spring over. Then I looked intently at the spring for a minute. I reassembled everything, took it for a ride and low and behold! I'm shifting in all four gears.
I can't explain how reversing the spring fixed it, I'm just happy to be one step closer to bringing this beast back from a long 35 year slumber.
With 3rd and 4th available I left the neighborhood for trips up and down the feeder road where I could stretch er legs a bit. Everything felt fine. 'Cept for a bit more top end noise after I got it really good and warm. I'll adjust the valves and take it out again tomorrow.
Oh! and the 51 year old Smiths speedo doesn't seem to want to register much over 40 mph. But Officer, I swear, I was only going 40......
Teaching my body to learn to operate a properly configured motorcycle is more difficult than I thought. Shifting with right foot is coming along. Teaching that left foot to brake is more of a challenge. Even after not riding for 30 some odd years, that brake reflex that my right foot developed is hard to unteach and I've downshifted right when I should have been on the brake left. Learning curve.
I recon learning to stop properly is a good reason not to venture forth into traffic until both the bike, and I, are sorted out.
Tomorrow I'll see if I can't figure a way to get the throttle cable sorted out. Then I can warm it up and tune the carb a bit. I haven't gone through the fine tuning procedure on the AMAL
Thanks all for the advice.