G'day, Neil,
On bikes without batteries it is common for the light to be dim at idle and then brighten up as the revs increase. Did the lights used to work better than they do now? Do you have the right bulb in there?
I used to have an American model '84 and still have an owner's and service manual. If you will pay for postage to New S. Wales, I'd be glad to mail it to you. Send me a private message if you want this manual.
The manual shows that the bike only had a 6 volt, 25 watt light. Not very bright. The wiring diagram is actually pretty simple, the yellow wire out of the magneto is the only one that powers the light, all the others are for the ignition.
The service manual has a trouble shooting section. For 'Jumps out of Gear' the items listed are 1) Shift fork worn 2) Driveshaft, out put shaft or gear splines worn, 3) shift drum groove worn, and 4) shift fork guide worn. I think 1) is the most common. Take note of which gears it pops out of most - that will help if you need to make transmission and shifter repairs.
The manual has a good section on transmission rebuilding.