'Is it advisable to replace the spokes with heavier spokes, if so, which diameter, is stainless steel better or not?'
i don't think it's nessesary to change the spokes to stainless. you can fit thin brass washers betwen spoke/hub (is this the right word?) and spokenut/ream. this will minimize tension. also, put plastic cable-binders (i hope you know what i mean) around each crossing of spokes. i never tried myself, but know some people who have good experience with these measures.
'Type - I cannot find the 3DS anywhere on the internet. It is from 1989. Any known problems with this type of Tenere?'
no, not realy. drive her like a ténéré should be driven and you won't get trouble. 3ds has a different carburator-adjustment for driving mainly above 1000 m above sea level.
'I opened up the engine of the 55W. Indeed, 3 of the five gears have serious pitting, so need to be replaced. Can I use the whole set
from another XT600, ie. both shafts and gears?'
according to the gears, you can use gears of any xt. but remember that the gear ratio has changed with 3AJ, so you have to adjust the secondary ratio (sprocket etc.).
you cannot use the shafts of xt e, they are longer so the chain-alignment won't fit.
avoid driving with low and very high revs.
use the proper engine-oil.
fit 14:40 secondary ratio.
'Also, the piston does not look new anymore. On the loaded side one stained spot. Also, oil scraper ring does not move freely in the
groove. Engine runs and starts well, no excessive smoke. Could I just replace the piston rings, or need to rebore and fit new piston?'
first measure the play between piston and cylinder (needs special device). if the play is above 0.08 mm (limit is 0.1 mm) you should rebore the cylinder and fit a bigger piston. important: don't trust anybody (means: measure the play again after reboring, should be 0.045 mm)!
if the play is okay, changing the piston rings should be enough. you might get a little higher oil-consumption afterwards.
regards
klaus@xt600.de