Thanks for all your replies.
Can someone define for me what a partial seizure is? I've come across it a lot while reading about these old bikes but I don't think I've got my head around it apart from the fact it is caused by the engine getting too hot, for whatever reason.
JeffS77: When you say "sucking vacuum" from somewhere are you talking about fuel flow issues or an air leak somewhere else in the system that causes this lean situation? As far as the jetting goes, the same mechanic (who is a nice guy and owns a little Honda 100 2 stroke of similar age but did also sell me the "it's a little engine, they get hot and need a rest" line) did recently tune and clean the carby and seemed to think all was OK there. Do you know where I can get a copy of Cycles South online? I'd love to watch it but I can't get Netflix here in Aus.
Twin-shocker: I guess I'll go and get another opinion somewhere. Same guy checked points (not sure about condensor), carb has been cleaned, the only thing is the exhaust hasn't been decarbonised because I think he said the muffler has been welded on or something at the very tip. When you say noisy top-end what am I listening for? It has been compression tested and it has plenty.
Is there anything a relatively mechanical noob like me can try? I'd like to get my hands dirty provided I don't get too out of my depth.