I hardly know my way around the engine, but I have been able to free a frozen piston on 3 bikes and get them running. It's a crap shoot, so procede carefully, which you seem to be doing.
Invest in an impact screw driver to free rusty screws. Deal with the tank and engine first and worry about trim and tires last. I have been able to use bicycle cables for brake and clutch cable replacements but they aren't as thick and the ends break easily if you have to keep loosenibg and retightening them . I would get a 4 pack of cables at Wal-Mart for $10, throw away the shortest cable as it won't fit anything and make sure I cut the correct end off so the other fits in the hand control.
Definetly get the clutch lever working first if you free up the piston because it's tuff to engage the clutch, pedal the bike, have your hand on the throttle, and start it by yourself. If the throttle cable is bad, probably best to buy a new one as I have never been able to cheat on that one.
Don't get discouraged. Soak stuff in penetrating oil and don't force things to the point that they break.