Ta very muchly for the advice dp.
Installed the belt drive a few days ago, with new cork washer as wot was missing, and all seems very well. Difficult to do up the dynamo pulley properly in situ, as remarked elsewhere in relation to some belt drive conversions. No endfloat on the idler shaft with all assembled, but you're probably right about the seal etc although there's no oil in the compartment and the dynamo itself was clean as a whistle inside. Most noises, and certainly the horrible ones, have now gone - the rest can wait till the whole engine comes apart as it surely will in due course (if only to reassure me as to what's in there on what is clearly a mix of bits).
Have been running 12 volts on another twin for over 20 years and nary a prob . . . that uses an original JG box of tricks from the '80s, and neither it nor the dynamo - 6 volt wound - have given any trouble in frequent use.
IMHO for what little it's worth 6v-wound is better, even if you need more revs to get it to balance the draw on the battery, cos the wires are thicker - the 12 volt windings are very very fine, obviously. Mine's kept the battery up even when I used the thing as daily transport in London for years. That runs a 60-55 halogen h/light. Was thinking of doing the same thing with the A10 using the Podtronics 12 volt box for 6v wound dynamos. Don't anticipate any probs, but fate does lurk round corners with a brick sometimes. Reckon it'd be better for the slightly skimpy loom on the bike too - I'd have used heavier gauge wire for a 6 volter than whoever wired this up, although so far nothing's melted. The coming winter will tell me more. Probably at the side of the road when it's wet and dark . . . Groily