Bob, lets answer your questions one at a time...
1)-"is there a conversion available for a John Deere 1943 Model A tractor from hand-crank start to electric start? It wears me out each time I have to hand-crank the engine over to start it"...>>> Answ, no actual kit available but if you search around you can find all the parts.. By far the best way to do that conversion is to buy an electric start tractor then swap over all the parts, then sell the that tractor as a hand start.. [sometimes a hand start will actually bring more money than an electric start]..
2)-"is the little clutch brake pad available? Is just the lining available or is it sold riveted to the metal backing?"...>>> Answ, yes, Deere has [or at least had them].. Kind or pricey for what you get.. You can always buy a clip on, then drill the rivets out, then buy new rivets from Deere & rivet that lining to your brake arm.. The holes don't always line up though but it will work with a couple of new holes..
3)-"when using the tractor, the High/Low gear selector lever pops out of gear quite regularly. What does this mean?"...>>> Answ, that could mean many things but the usual cause is worn trans bearings.. It could also mean that you have worn detents, or a bad bearing or wear on the trans input shaft..