all bearings and bushes are easily availiable, the actual gears should not give much bother. The guts are interchangable over many years, given a bit of common sense, selectors wear the worst, but pretty long lived.
I have a feeling a reversable camplate may once have been an option. How rare, or if they exist I don't know.
Personally down for up I found made for quicker gearchanges.
Have you consider a different rearset arrangement that would put the change as you want? rather than the simply reversed lever?
The long kick start looks to be off a later unit 650, but that cotter should be the other way round, and its too long, worth checking, if its a soft crap one it'll give trouble.