a 65 t120 should have the rocker inspection caps with screwdriver slots in them
these are exactly the correct size for the tappet adjuster tool to fit into, and with
this you can get get leverage and unscrew them.
the tool i have, which i believe is the factory tool, is a small flat plate
spanner, with an offset square slot at each end sized to fit the square
end of the tapped adjuster. sadly i cannot find a pictu of one onthe net.
the bottom line is you want somthing that fits the slot correctly to
unscrew a tight cap without damaging it.
the knurling on the rim of the cap are there to engage with a small strip of spring steel
and (hopefully) stop them unscrewing and getting lost, its not really strong enough to
get purchase on to unscrew a tight cap.
you shouldn't need oil, oil weeping is often a problem, if you need penatreaing oil if
you want such a thing wd40 is not much good, it's a. water dispersant, not a lubricant.
find some plus gas - try ebay, that is the real thing (tm).
i would also be surprised if the caps have corroded in place, there should be plenty
of oil about there, has the bike stood for a long time ( years?).
re heat. remember, you want to heat the rocker box and cool the cap, there are freezer
sprays you can buy now, or turn your butane blowtorch upside down and spray
some liquid butane on the cap which will cool it fast.
if you are still having problems leasing the boxes themselves this is even more worrying,
maybe somone has stuck them on without gaskets and using some hard setting sealant,
heamatite green perhaps? they should lift easily once all the bolts are undone.